Step 1: Roast the Squash and Garlic
Preheat the oven to 400°F (200°C). Halve the butternut squash lengthwise, remove seeds, and drizzle the cut sides with olive oil. Place cut-side down on a baking sheet. Slice the top off the head of garlic to expose the cloves, wrap it in foil, and place it on the sheet. Bake for 30-40 minutes until the squash is tender and the garlic is golden and soft.

Step 2: Cook the Pasta
While the vegetables roast, cook the paccheri pasta in a large pot of boiling salted water. Drain the pasta slightly before it reaches al dente, ensuring you reserve at least 1 1/2 cups of the starchy pasta water.

Step 3: Brown the Sausage
In a large Dutch oven over medium heat, cook the sliced sausage until deeply browned and cooked through. Remove the sausage with a slotted spoon and set aside, leaving the rendered fat in the pot.

Step 4: Sauté Aromatics and Paste
In the same Dutch oven, add the chopped onion and kosher salt. Squeeze the roasted garlic cloves out of their skins into the pot. Sauté for several minutes until the onion is translucent. Stir in the red pepper flakes, fresh sage, and tomato paste. Cook for 2-3 minutes until the tomato paste darkens in color.

Step 5: Create the Squash Sauce
Scoop the roasted butternut squash flesh from the skins and add it to the Dutch oven. Pour in 1 cup of the reserved pasta water. Mash the squash into the aromatics and stir until a smooth, velvety sauce forms. Add more pasta water if needed to reach your desired consistency.

Step 6: Combine and Finish
Add the cooked pasta, browned sausage, and grated Parmesan cheese to the sauce. Stir well to coat every piece of pasta. Serve immediately while hot and creamy.