You can add any ingredients to your frittata that you like (like an omelet). As a general rule, I add a combined amount of 1 cup of additional ingredients. (example: 1 cup combined of ham, tomatoes and onions. NOT 1 cup of each. Basically 1/3 cup of each.)
Find it online: https://aprettylifeinthesuburbs.com/three-cheese-frittata/