• Set a large pot of water to boil. Add a pinch of salt and cook the pasta 14 to 16 minutes until al dente. Drain and let cool slightly (you can run under cold water so they are cool enough to handle). Preheat the oven to 375℉.

For the Sauce

• Set a large skillet or pot over medium heat and saute the sausage with the onion. Break it up with a wooden spoon until very crispy and caramelized, about 15 minutes.
• Add the garlic, Italian seasoning, brown sugar, and spinach and stir a minute until fragrant, scraping up the bits from the pan.
• Add the canned tomatoes. Simmer about 10 minutes, stirring often to wilt in the spinach. Adjust the heat down as needed to keep it at a gentle bubble. It can simmer until you are ready to assemble.

For the Filling

• In a bowl, mix together the ricotta and cottage cheese, egg, salt, garlic powder and half the shredded cheeses.

To Assemble

• Ladle most of the sauce to a large 9x13 baking dish, saving about 1 to 2 cups. Use a spoon to fill each pasta shell with the cheese mixture and nestle into the sauce, making 3 rows of 7 shells.
• Add the rest of the sauce over the stuffed pasta, making sure enough of the shells peek out and are visible. Top with the remaining white cheddar, mozzarella and Parmesan cheese.
• Bake at 375℉ for about 45 minutes. Broil the top for a few minutes until golden and bubbly.
• Optional, garnish with fresh chopped parsley or basil.