Preheat broiler to high. In a medium bowl, whisk eggs, basil, milk, salt, and pepper.
In a 10" ovenproof nonstick skillet over medium-high, heat oil. Add bell pepper and onion and cook, stirring occasionally, until just tender, about 3 minutes. Push vegetables to one side of skillet and add sausage to the other side. Using a wooden spoon, break up sausage into small bits, then stir to combine with vegetables and cook, stirring occasionally, until sausage is golden brown and cooked through and vegetables are tender, 4 to 6 minutes.
Pour egg mixture into skillet with sausage and vegetables. Cook, slowly stirring with a spatula and scraping bottom and sides of skillet to form large curds, until eggs are almost set, 2 to 3 minutes. Spread into an even layer.
Transfer skillet to oven and broil until golden in spots and puffed, about 3 minutes. Remove from oven. Spread marinara over and sprinkle with mozzarella. Continue to broil until sauce is warm and cheese is melted and bubbly, 3 to 5 minutes more. Let cool about 5 minutes before serving. Sprinkle with basil.