Begin by cooking the rotini pasta until it is just under al dente, following the package instructions. This means we want it to hold its shape while cooking more in the slow cooker.
In a medium pan over medium heat, drizzle in the olive oil, then add the ground Italian sausage. Cook this for about 5 to 7 minutes, breaking it up into smaller pieces as it browns. Make sure to drain any excess grease.
In a large bowl, mix together the cooked pasta, browned sausage, salt, black pepper, onions, pizza sauce, cream of mushroom soup, and half of each cheese. Give it a good stir!
Next, grease the insert of your slow cooker. Pour the mixed ingredients into the slow cooker, spreading it out evenly.
Now it’s time to top it off! Sprinkle the remaining mozzarella, Colby jack, and Parmesan cheeses over the top.
Cover the slow cooker and cook on high for about 1 1/2 to 2 hours, or on low for 3 to 4 hours, until the cheese is melted and bubbly.
If you like, sprinkle some chopped fresh basil on top before serving for an extra pop of freshness and flavor.