Step 1

Position a rack in the middle of the oven and preheat it to 425 degrees. Whisk together the eggs and milk in a mixing bowl. Season lightly with salt and pepper.

Step 2

Brush a quarter sheet pan — or any shallow metal or ceramic 9-by-13-inch pan — with olive oil, line with parchment (leave enough overhang to help lift the frittata out later) and brush the paper with more oil. Scatter the onions and tomatoes evenly over the pan and season with salt. Bake for 15 minutes, until the onions have softened and darkened at the tips and the tomatoes have softened and started to collapse. Depending on what vegetables you use, you may need to reduce or increase the time.

Step 3

Pour the egg mixture over the roasted vegetables and scatter the roasted red pepper, spinach and brie over the surface. Bake for 20 minutes, until the eggs are just set and golden in spots. Lift the frittata out of the pan and onto a wire rack. Let cool slightly, cut into pieces and serve, or transfer to the refrigerator to store.