Preheat your oven to 375°F (190°C) and lightly grease a 9-inch pie dish.

Sauté finely chopped onions in a skillet over medium heat until translucent, about 3-5 minutes. Add heartier vegetables and cook until tender, around 5 more minutes. Allow to cool.

In a mixing bowl, whisk 6 large eggs until frothy, then gradually add 1 cup of whole milk, ½ cup half-and-half, kosher salt, black pepper, optional Dijon mustard, and nutmeg.

Fold in 1.5 cups of your chosen mix-ins into the egg mixture gently to maintain an airy texture.

Pour the mixture into the prepared pie dish and bake for 40-55 minutes until puffed and golden brown.

Let cool on a wire rack for 10-20 minutes before slicing. Serve warm or with a salad.