Preheat the oven to 375 degrees F (190 degrees C). Grease a 9-inch pie plate with cooking spray.

Melt butter in a medium cast iron skillet over medium heat. Add onion; cook and stir until softened and translucent, about 5 minutes. Stir in cherry tomatoes until slightly softened, about 3 minutes. Add mushrooms and cook until tender, about 3 minutes. Mix in spinach; cook for 1 minute more.

Whisk cream, eggs, salt, pepper, and nutmeg together in a bowl.

Spread 1/2 of the Gouda cheese in the prepared pie plate and top with spinach mixture. Cover with egg mixture and sprinkle remaining Gouda cheese on top.

Bake in the preheated oven until eggs are set and top is golden, about 25 minutes.