Preheat the oven to 350 degrees . Grease a 3-quart oval baking dish. In a large, heavy pot or dutch oven, heat the oil over medium heat. Add the leeks, onions and 1/2 teaspoon salt, cover and cook, stirring occasionally, for 10 minutes. Increase the heat to medium-high, uncover and cook, stirring frequently, until the vegetables are softened and browned, about 8 minutes; season with salt and pepper.
Line the bottom of the prepared baking dish with the toast and sprinkle with half of the cheese. Top with the onion mixture. Using the back of a spoon, make 8 deep wells in the onion mixture. Working 1 at a time, crack the eggs into a small bowl and pour them into the wells. Top with the remaining cheese; season with pepper.
Bake until the egg whites are set but still jiggly, about 20 minutes. Transfer the pan to the broiler and cook until the eggs are just set, 1 to 2 minutes.