Preheat your oven to 400°F and grease a 9x13 inch baking dish with butter or cooking spray. While the oven heats, prepare all your ingredients: dice the onion and red bell pepper into 1/4-inch pieces, finely mince the garlic, roughly chop the spinach, and have the crescent roll sheet ready at room temperature for easier unrolling.

Heat a large skillet over medium-high heat and add the breakfast sausage, breaking it into small crumbles as it cooks for 3-4 minutes until it's mostly browned and no pink remains. Add the diced onion, red bell pepper, and minced garlic, then cook for another 5 minutes, stirring occasionally, until the vegetables are tender and the flavors meld together. I like to cook the vegetables until they're just softened but still have a slight texture—this keeps them from becoming mushy in the casserole.

Remove the skillet from heat and stir in the chopped spinach, allowing it to wilt slightly from the residual heat of the sausage mixture. Unroll the crescent roll sheet into your prepared baking dish, pressing it gently to cover the bottom and sides evenly. Sprinkle 1/2 cup of the shredded cheddar cheese over the crescent layer, then spread the sausage and vegetable mixture evenly over the cheese.

In a large bowl, whisk together the room-temperature eggs, half-and-half, salt, and freshly ground black pepper until well combined and slightly frothy. This creates a smooth, creamy custard that will cook evenly throughout the casserole. Pour the egg mixture over the sausage layer, then sprinkle the remaining 1/2 cup of cheddar cheese on top. I recommend using freshly shredded cheese rather than pre-shredded, as it melts more smoothly and creates a better texture.

Place the casserole in your preheated 400°F oven and bake for 40-45 minutes, until the egg mixture is set but still slightly jiggly in the very center when gently shaken—it will continue to cook slightly as it cools. The top should be golden brown and a knife inserted near the center should come out clean or with just a tiny bit of custard clinging to it. Let the casserole rest for 5 minutes before serving, and garnish with the optional green onions if desired for a fresh, bright finish.