01 - Preheat oven to 350°F.
02 - In a large oven-safe skillet (preferably nonstick or cast iron), heat olive oil over medium heat. Add diced bacon and cook for 4-5 minutes, until golden and crisp.
03 - Add chopped onion and sauté for 2 minutes until softened. Stir in mushrooms and cook for another 4 minutes until tender and any liquid has evaporated.
04 - Add zucchini and garlic; cook for 2-3 minutes until zucchini is just tender.
05 - In a large bowl, whisk together eggs, milk or cream, cheese, parsley, salt, pepper, and thyme (if using).
06 - Pour egg mixture evenly over the sautéed vegetables and bacon in the skillet. Gently stir to distribute ingredients evenly.
07 - Cook on the stovetop over medium-low heat for 3-4 minutes, until the edges just begin to set.
08 - Transfer skillet to the preheated oven. Bake for 15-18 minutes, or until the frittata is puffed and the center is set.
09 - Remove from oven and let rest for 5 minutes. Garnish with extra parsley, slice, and serve warm or at room temperature.