Preheat the oven to 350°F (175°C). Spray a 9x13-inch baking dish with cooking spray and set aside.
In a large bowl, whisk together the 12 eggs, 1 cup sour cream, 1/4 cup milk, 2 cups shredded cheddar, 1 teaspoon kosher salt, and 1/2 teaspoon ground black pepper until just combined (an electric mixer on low is optional).
Heat a large skillet over medium heat and add the 2 pounds pork sausage. Cook, breaking into small pieces, until browned and cooked through. Drain most of the grease and transfer the sausage to the egg mixture.
Using the same skillet, add the diced 1/2 green bell pepper, diced 1/2 red bell pepper, and sliced 4 green onions; sauté for about 2–3 minutes until slightly softened. Add the vegetables to the egg and sausage mixture and stir to combine.
Pour the combined mixture into the prepared 9x13-inch baking dish and smooth the top. Bake at 350°F (175°C) for 35–50 minutes, until the edges are set and the center is just barely jiggly.
Remove from the oven and let rest a few minutes before slicing and serving.