Preheat the oven to 425. Spray a large baking sheet with nonstick cooking spray. Heat a large pot of water to cook the pasta.
In a medium bowl, toss the peppers, onions and garlic with the oil and salt. Spread the vegetables evenly on the baking sheet and roast them for 10 minutes, then flip them and cook them for 3 – 5 more minutes until they are peppers are browned in spots. Remove them from the oven and set them aside.
Meanwhile, cook the pasta according to the package directions, then transfer it to a large serving bowl and toss it with the roasted vegetables, the spinach, tomatoes, basil, vinegar and 2 Tbsp. of the oil from the mozzarella.
Toss it thoroughly, and when it has cooled a little, stir in the cheese. Season it with salt and pepper to taste and cover tightly and refrigerate it until you are ready to serve it, up to 2 days.