Heat the oven: Preheat to 350°F (175°C). Grease a 9-inch pie dish or a 9x9-inch baking dish with butter or olive oil.

Prep fillings: If using raw veggies like mushrooms or peppers, sauté them until soft and any moisture cooks off. Drain well. Chop cooked meats into bite-size pieces.

Whisk the custard: In a large bowl, whisk eggs until smooth. Add heavy cream, Parmesan, onion powder, salt, pepper, and nutmeg. Whisk until fully combined.

Add flavor boosts: Stir in garlic and fresh herbs if using. Fold in shredded cheese, cooked meat, and vegetables.

Assemble: Pour the mixture into the greased dish. Smooth the top and tap the dish gently to release air bubbles.

Bake: Place on the middle rack and bake 30–40 minutes, until the center is just set and the edges are lightly golden. A knife inserted near the center should come out mostly clean.

Rest and slice: Let the quiche rest 10 minutes before slicing. This helps it firm up and makes cleaner slices.

Serve: Enjoy warm or at room temperature. Add a simple green salad or sliced avocado on the side if you like.