Lightly grease the slow cooker insert with spray or a thin layer of olive oil or butter.

Add the chopped onion, broccoli, sliced carrots, sliced peppers, frozen peas, and halved cherry tomatoes into the slow cooker. Drizzle with olive oil.

Sprinkle the minced garlic, Italian seasoning, dried basil (if using), salt, black pepper, and red pepper flakes on top. Toss very gently to coat.

Pour the uncooked pasta over the vegetables. Do not stir — the pasta should sit on top.

In a medium bowl, whisk together the vegetable (or chicken) broth and heavy cream until combined. Stir in 3/4 cup grated Parmesan until it mostly dissolves.

Gently pour the broth-and-cream mixture over the pasta and vegetables. The liquid should just barely cover the pasta pieces.

Cover and cook on LOW for 2 1/2 to 3 hours, checking at about 2 hours 15 minutes and stirring gently halfway through.

Once the pasta is cooked, add the fresh baby spinach and fold it into the hot pasta and sauce until wilted — this takes 1–2 minutes.

Taste and adjust seasoning with extra salt, pepper, or red pepper flakes.

Sprinkle the remaining 1/4 cup Parmesan over the top, cover for 5 minutes to let flavors meld, then serve in warm bowls with parsley or extra spinach and more Parmesan at the table.