Super Easy Pesto Peppers And Penne Skillet

2 cups penne pasta
1 Tbsp olive oil
1 pkg (16 oz) frozen mixed bell peppers and onions
1 pkg (10 oz) basil pesto
reserved pasta water
salt and pepper to taste
fresh grated Parmesan cheese for garnish

Bring to boil a large pot of salted water, add penna, and cook (don't boil too hard) until just tender; drain into a bowl - you'll need some of the reserved pasta water later.
Meanwhile, put a large heavy skillet on medium heat, add olive oil and pepper/onion mixture, stirring until crisp-tender, not soft. Add the drained pasta and pesto to the skillet, turn heat down to low, and stir until mixture is heated through.
Add 1/2 cup of the reserved pasta water, stir until a sauce develops.  If you want more sauce, add more reserved pasta water, and continue to stir.
When you're happy with the sauce, continue cooking over low heat just until everything is hot.
Serve hot in pasta bowls, grating Parmesan cheese over each serving.
Serves 4 to 6 people.