Preheat oven to 400°F(200°C). Lightly grease a shallow casserole dish.
In a skillet over medium heat, cook the sausage and onions, breaking up the pieces as it cooks, until browned. Drain any excess grease from the pan.
Add the garlic and sauté for just 30 seconds, until aromatic.
Add marinara sauce, Italian seasonings (basil, thyme, oregano), and salt.
Simmer for 5 minutes, stirring occasionally. Add 1 cup of the mozzarella and stir it in.
Spread mixture into the bottom of the prepared dish. Add the ricotta in spoonfuls around the dish, don’t stir it in. Top with the remaining mozzarella.
Bake for 10 minutes, or until hot and bubbly. Garnish with fresh basil if desired.
Serve with bread, crostini, or breadsticks.