rigatoni with grilled peppers and onions

This salad was SOOOO GOOD!

rigatoni with grilled peppers and onions (Cassie Craves got it from Real Simple) 
2 medium red onions, sliced into rings (I simply halved mine)
2 large sweet bell peppers, cut into quarters
olive oil
kosher salt and pepper
12 ounces (4 to 5 cups) rigatoni
1 bunch spinach, stems trimmed, or one 5-ounce package
1 cup grated Parmesan
3/4 cup basil leaves, thinly sliced 

Bring a large pot of water to a boil.  Heat a grill or grill pan to medium-high.  In a large bowl, toss the onions, bell peppers, oil, 1/2 teaspoon salt and 1/2 teaspoon pepper.  Grill, turning occasionally, until tender, 8 to 10 minutes.  Transfer to a cutting board and cut into bite-sized pieces.
Cook the pasta according to the package directions.  Reserve 1 cup of the cooking water, drain the pasta and return the pasta to the pot.  Add the grilled vegetables, spinach, the reserved pasta water and 3/4 cup of the Parmesan to the pasta and toss to combine.  Top with the basil and the remaining Parmesan before serving