Preheat the oven to 375°F (190°C).
To a large skillet over medium heat, add olive oil. Once hot, add sausage, onion, ½ teaspoon Italian seasoning, salt, and pepper. Cook for about 8 minutes, breaking up the sausage with a spoon, until the sausage is fully cooked.
Add the garlic and cook for an additional minute.
Stir in marinara sauce and bring to a simmer. Once simmering, remove the skillet from heat.
In a medium bowl, combine the ricotta cheese, ½ cup of shredded mozzarella, parmesan, the remaining Italian seasoning, and red pepper flakes (if using). Stir until well mixed.
Spread the ricotta mixture evenly in the bottom of a small oven-safe skillet or baking dish.
Add the sausage mixture evenly on top of the ricotta layer. Sprinkle the remaining 1 cup of shredded mozzarella cheese evenly over the sausage layer.
Bake for 15-20 minutes, or until the cheese is bubbly and golden.
Remove from the oven and garnish with chopped fresh basil. Serve hot with breadsticks, crostini, or crackers for dipping.