Preheat the oven to 325°F, and set an oven rack in the middle position.
Heat the oil in a 10-inch ovenproof nonstick skillet over medium heat.
Add chopped onions to the skillet and cook, stirring often, until soft, about 5 to 7 minutes.
Add your precooked vegetables to the skillet and stir well to combine them with the onions.
In a large mixing bowl, combine the eggs, heavy cream, Knorr chicken-flavored bouillon (or kosher salt), and pepper. Whisk these ingredients together until they are well blended.
Add sautéed vegetables and cheese to the mixing bowl, stirring thoroughly for even distribution.
Pour the egg and vegetable mixture back into the same skillet.
Carefully place the skillet into your preheated oven and bake for 20 to 23 minutes, or until the frittata is set and cooked through.
Serve the frittata from the pan or slide it onto a platter using a spatula.