Dice the onion into 1/4-inch pieces, slice the mushrooms, chop the bell peppers into 1/2-inch pieces, roughly chop the spinach, and mince the garlic cloves. Cut the bread into 1-inch cubes and set aside. Having everything prepped and ready will make the cooking process smooth and allow you to build layers of flavor without rushing.

Heat 1 teaspoon of olive oil in a large skillet over medium-high heat. Add the ground pork sausage and cook for 5-6 minutes, breaking it apart with a spoon as it cooks until it's no longer pink. Remove the cooked sausage and set aside on a plate. In the same skillet with the rendered fat, add the diced onion and cook for 2-3 minutes until softened, then add the mushrooms and bell peppers and cook for another 3-4 minutes until they begin to soften. Add the minced garlic and dried rosemary, stirring for about 30 seconds until fragrant. Finally, add the chopped spinach and stir until wilted, about 1 minute. Return the cooked sausage to the skillet and stir everything together to combine. This method builds deep flavor by letting the vegetables soften in the sausage fat and toasting the herbs.

Grease a 9x13-inch baking dish lightly. Spread the bread cubes evenly across the bottom of the dish, then pour the sausage and vegetable mixture from Step 2 over the bread, spreading it out evenly so the filling is distributed throughout.

In a large bowl, whisk together the 12 eggs, 2/3 cup milk, 1 cup shredded cheddar cheese, 1/4 teaspoon salt, and 1/8 teaspoon black pepper until well combined. Pour this egg mixture evenly over the bread and sausage mixture in the baking dish, using a fork or spatula to gently push down on the bread so it absorbs the custard. I like to let it sit for a minute and then give it another gentle stir to ensure the bread is saturated with the egg mixture.

Cover the baking dish with plastic wrap or foil and refrigerate for at least 30 minutes, or up to overnight. This resting time allows the bread to fully absorb the egg custard and helps the flavors meld together. When you're ready to bake, remove the dish from the refrigerator and let it sit at room temperature for about 10 minutes while you preheat the oven.

Preheat the oven to 375°F. Remove the cover from the baking dish and place it in the preheated oven. Bake for 40-45 minutes, until the top is golden brown and a knife inserted in the center comes out clean or with just a few moist crumbs. The egg custard should be set but still slightly tender in the center.

Remove the egg bake from the oven and let it cool for about 10 minutes before slicing. This resting period allows the custard to set up a bit more and makes it easier to cut clean portions. Garnish with fresh chopped parsley and green onion if desired for a pop of fresh flavor and color. Slice into squares and serve warm.