If you want to make it believable you can build your diets based on these:
- available ingredients;
- available cooking technologies;
Your people live somewhere and they get foodstuffs somehow. What is the climate of their area? What plants grow in this climate? What animals live there? You can check types of existing biomes and then look them up to see what lives there. Then see what kind of plants and animals locals consume for food. If your flora and fauna are not terrestrial researching biomes will give you some ideas about features typical for plants and animals living in specific climates.
The next series of questions for this part deals with lifestyle. Are your people hunters, gatherers, farmers, fishers? Do they migrate with seasons? Do they trade with other peoples?
Cooking techniques and technologies
Food can be boiled, fried, baked, fermented, pickled, frozen, served fresh, etc. Check Wikipedia for specific techniques and their descriptions. Choice of cooking techniques depends on technology (do your people have ovens and frying pans?), ingredients, and lifestyle. For example, pickling would be widespread in societies that have short growing seasons, no greenhouses, and easy access to salt. Deep frying would be rare if oils and fats are not easy to get. You can check the historical period and culture to see what combination of techniques was most prevalent in specific circumstances.
Culture may dictate what can and cannot be eaten. For example, it may place a taboo on eating meat or drinking alcohol despite both being widely available. You should consider, though, that most taboos are practical and they will not cause starvation. It is impossible to forbid meat in climates where people rely on meat to provide essential nutrients.
Culture also affects preparation and presentation. For example, both Chinese and Japanese cuisines cut food into pieces that are easy to eat with chopsticks. However, Chinese cuisine manipulates ingredients and their taste to a greater degree than Japanese cuisine, because their respective cultures appreciate the natural taste of an ingredient in different ways.
Once you are done with figuring out what your people can possibly cook and how they do it, you can try to make it more 'interesting'. Add some insects for texture and crunch. Use some water plants (kombu, for example) for decoration. Make your people admire the weightlessness of souffle or a pate. Create a culture that serves a lot of side dishes so you can showcase your effort. Or go the other way and make it very simple and practical.