Preheat the oven to 350°F (175°C). Lightly grease a deep pie dish or a 9x13-inch rectangular baking pan to prevent sticking.
Warm a large skillet over medium heat. Add the ground sausage and cook, crumbling it with a spatula, until browned throughout and no pink remains. Drain off any rendered fat and set the cooked sausage aside.
Distribute the sausage evenly over the base of the greased baking dish. Scatter the chopped onion and optional green bell pepper across the surface. Sprinkle both varieties of shredded cheese uniformly over the vegetables and meat.
In a mixing bowl, vigorously whisk together the eggs, half-and-half, flour, baking powder, salt, and black pepper until the mixture is smooth and lump-free.
Carefully pour the egg mixture over the cheese-topped sausage base, ensuring even coverage across the pan.
Place the dish in the preheated oven and bake for 35 to 40 minutes, or until the center is firm and the surface develops a golden hue.
Remove from the oven and allow the pie to cool slightly before cutting into portions. Garnish with green onions or chives if desired, and serve warm.