Step 1

Arrange a rack in the middle of the oven and heat to 375°F. Coat a 9x13-inch baking dish with olive oil.

Step 2

Heat the oil in a large frying pan over medium heat until shimmering. Add the onion, pepper, and 1/2 teaspoon of the salt. Cook, stirring occasionally, until softened, about 5 minutes. Add the spinach and garlic, and toss until wilted and almost all of the liquid is evaporated, about 3 minutes. Remove from the heat and stir in the dill. Transfer to the baking dish and spread in an even layer.

Step 3

Place the eggs, milk, mustard, remaining 1 teaspoon salt, and pepper in a large bowl and whisk until the eggs are completely broken up and incorporated. Pour over the spinach mixture in the baking dish. Sprinkle evenly with the feta.

Step 4

Bake until the top is light golden-brown and a knife inserted in the middle comes out clean, 40 to 45 minutes. Let cool for 5 minutes before slicing.