Preheat your oven to 375°F (190°C). Grease your baking dish with a little extra olive oil to prevent sticking.
In a large skillet, heat 1 tablespoon of olive oil over medium heat. Add the chopped Yukon Gold potatoes along with ½ teaspoon of sea salt and freshly ground black pepper. Cook for about 10-15 minutes, stirring occasionally, until the potatoes are tender and lightly golden.
Add the chopped onion and garlic to the skillet with the potatoes. Sauté for another 3-4 minutes until the onions become translucent. Then, add the chopped red and green bell peppers, and continue to cook for an additional 5 minutes. Finally, stir in the fresh spinach and cook until wilted. Remove from heat and set aside.
In a large mixing bowl, whisk together the 12 large eggs, 1 cup of milk, and 1 teaspoon of sea salt until well combined.
Add the sautéed vegetable mixture to the bowl with the egg mixture. Gently fold in the chopped green onions and shredded cheddar cheese until everything is evenly distributed.
Pour the egg and vegetable mixture into the prepared baking dish, spreading it out evenly. Ensure that the ingredients are well distributed throughout the dish.
Place the baking dish in the preheated oven and bake for 30-35 minutes, or until the casserole is set and the top is lightly golden. A knife inserted in the center should come out clean.
Allow the casserole to cool for a few minutes before slicing it into squares. Serve warm, and enjoy the hearty flavors of this Breakfast Casserole!