Step 1

Preheat the oven to 425F.

Step 2

Heat the oil in a medium cast iron pan or oven-proof skillet over medium heat.

Step 3

Add the onions and cook for 5 minutes, stirring often, until softened.

Step 4

Add the spinach and cook until just wilted, about 2 minutes.

Step 5

Pour the eggs over.

Step 6

Scatter the tomatoes, mozzarella, and basil on top (they should sink slightly).

Step 7

Lower the heat slightly and cook until mostly set, about 5 minutes.

Step 8

Transfer to the oven and bake 5 to 10 minutes (ovens vary so much! It took me 9 minutes), until fully set and cooked through.

Step 9

Remove from the oven and allow to cool slightly.

Step 10

Cut into slices and serve (or refrigerate overnight).