Place one spaghetti squash on a large cutting board, cut off the stem end with a chef’s knife, then stand it upright and carefully slice it in half from top to bottom; repeat with the second squash.

Use a spoon to scoop out the seeds and stringy center from each half, leaving a clean cavity in the middle.

Line a baking sheet with parchment paper, then place the squash halves cut side up on the sheet.

Drizzle 2 tbsp olive oil over the cut sides with a measuring spoon and sprinkle 1 tsp kosher salt and ½ tsp black pepper evenly over the surfaces.

Use your hand to rub the oil and seasoning across the cut sides so they are lightly coated, then turn the halves cut side down on the baking sheet.

Put the baking sheet on the middle rack of a preheated 400°F oven using oven mitts and roast for 35 minutes until the shells give slightly when pressed and the flesh feels tender when pierced with the tip of a knife.

While the squash roasts, set a large skillet on the stove over medium heat and add the Italian sausage.

Break the sausage into small pieces with a wooden spoon and cook for 7 minutes, stirring often, until the sausage is browned and no pink remains.

Add the chopped onion to the skillet and cook for 4 minutes, stirring with the wooden spoon, until the onion softens.

Stir in the minced garlic and cook for 1 minute until fragrant.

Pour 1 cup crushed tomatoes or tomato sauce into the skillet with a measuring cup, then add 1 tsp Italian seasoning, ½ tsp salt, and ¼ tsp pepper; stir with the wooden spoon until everything is combined.

Let the sausage mixture simmer on low heat for 5 minutes so the flavors blend, then turn off the heat.

When the squash is done, remove the baking sheet from the oven with oven mitts and let the halves cool for 5 minutes on the sheet.

Turn the squash halves cut side up with a spoon and use a fork to scrape the inside of each half, pulling the flesh into long spaghetti-like strands while leaving a thin layer attached to the shell for strength.

Transfer most of the strands into a large mixing bowl, keeping the shells on the baking sheet to use as bowls.

Pour the sausage mixture from the skillet into the bowl with the squash strands and stir with the wooden spoon until everything looks evenly mixed.

Spoon the sausage and squash mixture back into each squash shell on the baking sheet, packing it gently and mounding the filling slightly in each half.

Sprinkle 1½ cups shredded mozzarella evenly over the tops, then scatter ¼ cup grated Parmesan over the mozzarella.

Return the baking sheet to the oven on the middle rack and bake at 400°F for 15 minutes until the cheese is fully melted and starting to brown in spots.

Take the baking sheet out with oven mitts and let the sausage stuffed spaghetti squash rest for 3 minutes, then use a large spoon to transfer each squash half to a plate and serve hot as a main dish.