Sometimes recipes are protectible as "trade secrets," but CA state law says the secrets have to be actual valuable secrets not known in the industry that the owner has taken steps to protect from disclosure, and in this case, it's unclear who the owner is. If it's a family recipe, it may belong to your uncles, too, are it's unclear how your uncles would be able to sell it and why anyone would pay for it.
Generally food is protected by trademark, not by trade secret law, although sometimes companies have both, like Coca Cola's cola recipe. Recipes aren't protected by copyright law unless the expression of the recipe is creative and mnore than just a list of ingredients, such as in poetry, with graphics, in a collective work arranged in a copyrighted way, etc. etc
You state that your father "bought the recipe years" and maybe you mean "years ago," but from whom did he buy it? If the recipe was known to your uncles, then maybe it wasn't a secret.
You need to consult a lawyer to fully disclose the facts about your situation.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
You could try to obtain a utility patent on the recipe. It is sometimes possible to obtain utility patents on recipes though it is tricky.
You could file a trademark application on the name of the dish, though this will only protect the name.
There is trade secret protection if you keep the recipe secret. But if the recipe is reverse engineered or someone independently comes up with something similar, trade secret protection will probably not protect you.
You can have employee agreements that require confidentiality or can have employees sign confidentiality agreements.
In addition to trademark and trade secret protection, there are times when someone who creates a recipe can obtain patent protection for it. For example, many prominent companies who produce candy bars (such a Mars) regularly apply for and obtain patent protection for new recipes and/or methods of producing their products. However, in this case patent protection probably is not available since it looks like the recipe was sold to your father years ago, in which event it is probably too late to seek patent protection. Normally you need to file a patent application within a year of the first sale or offer for sale.
It sounds to me that in this situation, your only hope would be to assert that the recipe is a trade secret. However, trade secret protection is hard to establish. You will need to establish that your father took active steps to preserve confidentiality---and this is often difficult to prove. Indeed, it seems that he may have disclosed the recipe to your uncles, which may be sufficient to result in loss of confidentiality.
You also will need to prove that there truly is something special about this trade secret. Courts are very skeptical about claims that food recipies are trade secrets---chances are someone out there has published the identical recipe or something very similar to it. If someone has published the same or substantially similar recipe, you cannot claim that your recipe is a trade secret.
You would also need to prove that you own the recipe. That may not be so easy if your father allowed your uncles to use it in the past. Your uncles may appropriately claim an ownership interest in the recipe, in which event they may be fully within their rights to sell their ownership interest. Alternatively, if your father was the owner of the recipe, but if he disclosed it to your uncles, a court would probably find that your father did not take sufficient steps to maintain confidentiality, and therefore the recipe does not constitute a trade secret.
Finally, you need to be practical. It is rare that people can successfully claim that a food recipe is a trade secret. Further, it is not clear to me that sale of the recipe by your uncles would necessarily damage your ability to operate a restaurant in the future. Indeed, if as I suspect, the recipe no longer qualifies as a trade secret, you will be free to use it in your restaurant even if your uncles "sell" the recipe. It would likely be very hard to convince a court to block your uncles' proposed sale of the recipe, nor would you likely convince a court that you have been damaged.
Nonetheless, I do not have suffiicient information about the facts and circumstances to give you definitive advice on this question. Thus, you should consult with an IP lawyer on this matter.
When you say "protect" I suspect that you are seeking to prevent your uncles, who apparently know the secret recipe, from selling it to someone else, and depriving you from exclusively using it in a restaurant you are planning to open. It is unclear from your question whether you already have a copy of the recipe or whether you wish to get it from them.
It seems to me that your need is to negotiate with them for a course of action that is in everyone's best interests. I suggest you seek a counselor who is skilled in mediation.
The foregoing is not legal advice. You should seek such advice from an attorney admitted to practice in your jurisdiction to whom you confidentially disclose the details of the matter.