Preheat oven to 400° F. Whisk together eggs, half-and-half, salt and pepper in a mixing bowl until completely mixed and light-yellow. Set aside.
Heat olive oil in large oven-safe, cast-iron skillet over medium heat. Add onion and mushrooms to the skillet. Cook, stirring often until onions are translucent and start to brown. Drain off any excess liquid and add chopped kale. Cook 3–5 minutes more until softened, stirring often.
Add feta and egg mixture to same skillet. Cook on medium heat for 5–7 minutes, or just until eggs start to set. Scrape bottom and stir with spatula constantly to prevent sticking. Remove skillet from heat and insert tomatoes into the frittata, keeping some of the tomato exposed.
Bake at 400° F for 10–12 minutes, or until cooked through and just firm. Cool slightly before slicing. Store leftovers sealed in the fridge for up to a week.