Heat olive oil in an oven-safe skillet over medium heat. Add the mirepois mix and sauté until softened, about 5-7 minutes. Season with salt and pepper to taste.
In a mixing bowl, whisk together eggs, cheese and milk until well combined. Season with a pinch of salt and pepper.
Pour the whisked eggs over the sautéed mirepoix vegetables in the skillet. Stir gently to distribute the vegetables evenly in the eggs.
Cook on the stovetop over medium heat for 3-5 minutes, or until the edges of the frittata start to set.
Transfer the skillet to the preheated oven and bake for 15-20 minutes, or until the frittata is set in the middle and has a golden brown top. Serve hot.
