Roast the potatoes: Preheat the oven to 425°F and line a baking sheet with parchment paper. Place the potatoes on the baking sheet. Drizzle with the olive oil and sprinkle with the salt and several grinds of pepper. Toss to coat. Spread evenly on the baking sheet and roast for 20 to 25 minutes, or until tender and browned.

Make the casserole: Reduce the oven temperature to 350°F and oil a 9x13-inch baking dish.

In a large bowl, whisk together the eggs, milk, and 1/4 teaspoon of the salt. Set aside.

Heat the olive oil in a large skillet over medium heat. Add the onion, the remaining 1 teaspoon salt, and several grinds of pepper. Cook, stirring occasionally, for 5 to 8 minutes, or until soft. Add the garlic, red pepper, and green pepper and cook, stirring occasionally, for 2 minutes. Add the spinach and half the green onions and toss until the spinach is just wilted.

Spread the potatoes in an even layer in the prepared baking dish. Top with 1 cup of the cheese, followed by the sautéed veggies. Pour in the egg mixture and sprinkle with the remaining 1/2 cup cheese and the reserved green onions.

Bake for 40 to 45 minutes, or until the eggs are set.

Let stand for 5 minutes before slicing and serving.