Heat oven to 350° F.
Add the oil to a 2-quart casserole and transfer to oven for 5 minutes.
Meanwhile, in a bowl, whisk together the eggs, salt, and pepper. Add the spinach, tomatoes, and scallions and combine. Gently stir in the feta.
Remove casserole from oven. Pour the egg mixture into casserole. Bake until the frittata is browned around the edges and slightly puffed and a knife comes out clean, 25 to 30 minutes.