Slice the wild mushrooms into even pieces and set aside. Preheat your oven to 180°C (350°F).
Heat olive oil in an ovenproof skillet over medium heat until shimmering. Add the sliced mushrooms and sauté for about 5-7 minutes, stirring occasionally, until they turn golden and fragrant.
Add the minced garlic to the mushrooms and cook for another 30 seconds until fragrant, taking care not to burn it. The mushrooms should smell nutty and smoky now.
While the mushrooms cook, whisk the eggs in a mixing bowl with the milk, salt, pepper, and grated cheese if using, until the mixture is frothy and well combined.
Pour the beaten eggs evenly over the sautéed mushrooms in the skillet. Gently shake the pan to distribute the eggs and mushrooms evenly, then let cook on the stovetop for 2-3 minutes until the edges just start to set and turn lightly golden.
Sprinkle a little extra cheese on top if desired, then transfer the skillet directly into the preheated oven. Bake for 12-15 minutes until the frittata is puffed, golden, and the center is just set.
Remove the skillet from the oven and let the frittata rest for 5 minutes. The residual heat will finish setting the eggs and meld the flavors.
Garnish with freshly chopped herbs and slice into wedges. Serve warm or at room temperature to enjoy the earthy aroma and creamy texture of this rustic mushroom frittata.