Mix together the ricotta, Parmesan, egg, garlic, 1/2 teaspoon salt, a few grinds of pepper and 1 cup of the mozzarella in a medium bowl until combined.
Spread 1/3 cup of the cheese mixture in the bottom of an 8 1/2-by-4 1/2-inch glass loaf pan. Top with 1 lasagna noodle, followed by one-third of the spinach, one-third of the red pepper and then one-third of the sausage. Repeat the layering twice, starting with 1/3 cup of the cheese mixture. Top with the last lasagna noodle and spread with the remaining cheese mixture. Sprinkle with the remaining 1/2 cup mozzarella.
Bake until the top is golden brown and bubbling and the noodles are cooked through, 25 to 30 minutes. Let cool for 10 minutes before serving.
This dish can be assembled up to 24 hours in advance, then covered and refrigerated. When ready to cook, bake according to the recipe directions.
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