Start with a large skillet or braising pan set over medium heat on the stovetop. Add a little oil and place 1-pound of breakfast sausage in the pan. Using a wooden spoon or fork, work on breaking up the sausage and moving it around.

Continue breaking it up and stirring it as it cooks until no pink spots remain and the sausage is crumbled into smallish pieces, about 8-10 minutes. Transfer the sausage to a paper towel lined plate to drain off some of the fat and set aside for now.

This next part can be done in two different ways, so it’s your choice here. Add the hashbrowns to the dish raw or precook them. Both ways are fine, but I like to toast them up a bit myself.

Mix 1 teaspoon kosher salt and ½ teaspoon ground black pepper together and set aside. In the same pan you cooked the sausage, melt ¼ cup of unsalted butter and then add 3 cups of frozen hashbrowns and turn the heat to medium-high. Add a pinch or two of salt and pepper, stir the hash browns so they’re coated in the melted butter, and add another pinch of salt and pepper.

Now spread out the hashbrowns into a single, even layer and let them cook for 5 minutes without disturbing them. Flip them over and cook for another 5 minutes to brown on the other side. Give them a stir and transfer them to a 13x9 baking dish or a deep casserole dish and set them aside.

If you don’t want to pre-cook the hashbrowns, you should thaw them overnight in the refrigerator the day before or leave them out at room temperature for an hour or so. Dump them onto some paper towels and use more paper towels to dry them off as much as possible. Now add them to the bottom of the baking dish as above.

Add some more butter or oil to your skillet if needed and now add the diced ½ yellow onion. Sauté for 2-3 minutes, then add the diced medium green and red bell pepper to the skillet. Season with a pinch or two of salt and pepper and continue cooking until the onions turn translucent and the peppers have softened. About 5-6 minutes longer.

Finally, add the 2 minced cloves of garlic, the cooked sausage, and 4-ounces of baby spinach to the pan. Stir everything together and cook for 1-2 minutes, until the spinach has wilted and shrank. Set everything aside to cool down a bit.

In a large mixing bowl, add all 12 large eggs and whisk together until there’s no visible streaks of yolks or whites. Pour in ⅔ cup half-and-half (or other milk/cream), the remaining salt and pepper, and the ½ teaspoon mustard powder. Whisk again until fully combined and set aside.

Now pour the egg batter evenly over everything else. From here, you can cover and refrigerate overnight or cook immediately. If you use bread instead of hashbrowns, I recommend the overnight soak. Using potatoes makes this optional.

Preheat the oven to 350°F / 180°C.

If cooking the casserole immediately, cover the baking dish with foil and place onto center rack in the oven. If cooking it the next morning, let it sit at room temperature for 15-20 minutes before baking.

Bake, covered, for about 40 minutes. Remove from the oven and uncover. Sprinkle on the remaining cheese and return to the oven. Continue baking for another 15-20 minutes. The center should just barely jiggle, and the cheese should be melted and slightly browned.

Remove from the oven and set aside to cool a bit. You should give it at least 15 minutes and up to 30 minutes or more to cool and set.

Optionally top with some freshly chopped parsley or green onion, cut into 12 portions, and serve warm.