1-Preparing the Potatoes: Start by preheating your oven to 425°F and lining a baking sheet with parchment paper. This prevents the potatoes from sticking and makes cleanup easier. While the oven heats, chop your Yukon Gold potatoes into uniform ¼-inch pieces. This small size is important because it allows the potatoes to become tender and develop crispy edges without requiring excessive roasting time.

2-Preparing the Potatoes: In a mixing bowl, toss the chopped potatoes with 1 tablespoon of olive oil, ½ teaspoon of the salt, and black pepper to your taste. The oil helps the potatoes become golden and crispy, while the seasoning enhances their natural flavor. Spread the seasoned potatoes evenly in a single layer on the prepared baking sheet, ensuring they aren’t overcrowded, which would cause them to steam rather than roast.

3-Preparing the Potatoes: Place the baking sheet in the preheated oven and roast for 20-25 minutes, or until the potatoes are tender inside and nicely browned on the outside. You’ll know they’re ready when you can easily pierce them with a fork and they have developed a slight crispness around the edges.

4-Preparing the Egg Mixture and Vegetables: While the potatoes are roasting, reduce your oven temperature to 350°F in preparation for baking the assembled casserole. Grease a 9×13-inch baking dish with a little olive oil or cooking spray to prevent sticking.

5-Preparing the Egg Mixture and Vegetables: In a large mixing bowl, whisk together 12 large eggs, 1 cup of milk (any unsweetened variety works fine), and ¼ teaspoon of the salt. Whisk until the mixture is well combined and slightly frothy, which helps create a lighter texture in the finished casserole. Set this egg mixture aside while you prepare the vegetables.

6-Preparing the Egg Mixture and Vegetables: Heat 2 tablespoons of olive oil in a large skillet over medium heat. Add the chopped yellow onion along with the remaining 1 teaspoon of salt and black pepper to taste. Sauté for 5-8 minutes, stirring occasionally, until the onions have softened and become translucent.

7-Preparing the Egg Mixture and Vegetables: Add the minced garlic, chopped red bell pepper, and chopped green bell pepper to the skillet. Continue cooking for another 2 minutes, allowing the peppers to soften slightly but still retain some crispness. The garlic will become fragrant, which signals it’s time to move to the next step.

8-Preparing the Egg Mixture and Vegetables: Finally, stir in the fresh spinach and half of the chopped green onions. Continue cooking for just 1-2 minutes, until the spinach wilts. Remove the skillet from heat, as the vegetables are now ready to be assembled with the other ingredients.

9-Assembling and Baking the Casserole: Once your potatoes are roasted and your vegetables are sautéed, it’s time to assemble the casserole. Start by spreading the roasted potatoes evenly across the bottom of the greased baking dish. This creates a sturdy, flavorful base for the other ingredients.

10-Assembling and Baking the Casserole: Sprinkle 1 cup of the shredded cheddar cheese over the potatoes. The cheese between the potato layer and vegetables helps bind everything together and adds delicious pockets of melted cheese throughout the casserole.

11-Assembling and Baking the Casserole: Evenly distribute the sautéed vegetable mixture over the cheese-topped potatoes. Use a spatula to spread the vegetables in an even layer, ensuring that all parts of the dish will have a balanced mixture of ingredients.

12-Assembling and Baking the Casserole: Gently pour the egg mixture over the vegetables and potatoes. The egg mixture will seep down through the layers, which is exactly what you want. Take a moment to ensure the egg mixture distributes evenly across the dish.

13-Assembling and Baking the Casserole: To finish assembling, sprinkle the remaining ½ cup of shredded cheddar cheese and the reserved green onions over the top of the casserole. The top layer of cheese will become beautifully golden and bubbly as it bakes, while the green onions add a fresh finish and pop of color.

14-Assembling and Baking the Casserole: Place the baking dish in the preheated 350°F oven and bake for 40-45 minutes. The casserole is done when the eggs are completely set in the center. You can test this by inserting a knife into the middle of the casserole if it comes out clean, the eggs are set. If there’s wet egg mixture on the knife, continue baking for a few more minutes and test again.

15-Assembling and Baking the Casserole: Once the casserole is done baking, remove it from the oven and let it rest for 5 minutes before slicing and serving. This resting period allows the eggs to set fully and makes the casserole easier to cut into clean portions. For a beautiful presentation, garnish with additional fresh green onions or herbs before serving.