Preheat your oven to 190°C (375°F). Gather an ovenproof skillet, a sharp knife, a cutting board, a mixing bowl, and a whisk to prepare for cooking.
Thinly slice the mushrooms and finely chop your herbs. Crack the eggs into the mixing bowl, add a pinch of salt and pepper, then whisk gently until just combined for a light, airy custard base.
Heat the butter or olive oil in your ovenproof skillet over medium heat until shimmering, and add the sliced mushrooms. Cook, stirring occasionally, until they turn golden brown and smell nutty, about 8-10 minutes.
Reduce the heat to low. Pour the beaten eggs evenly over the caramelized mushrooms in the skillet. Gently shake or tilt the pan to distribute the eggs evenly, then cook for 2-3 minutes until the edges start to set with a slight crackle.
Sprinkle the grated cheese evenly over the eggs, then scatter your chopped herbs on top. Transfer the skillet to the preheated oven and bake for 12-15 minutes, until the frittata is puffed and golden, and the eggs are just set.
Check that the surface is golden and the eggs are cooked through—gently shake the skillet; the frittata should jiggle slightly but hold together. If needed, bake a few more minutes or cover loosely with foil to prevent over-browning.
Remove the skillet from the oven and let the frittata rest for 3-5 minutes. This helps it set and makes slicing easier. Serve warm directly from the skillet or transfer to a plate, garnished with extra herbs if desired.
