Pesto Spaghetti Squash Sausage Skillet

medium spaghetti squash (cut in half and seeds discarded)

olive oil (divided)

Salt and pepper

sweet pepper (diced)

medium onion (finely diced)

chicken or turkey sausages (sliced 1/4-in thick)

prepared basil pesto

Shredded Parmesan cheese for topping (optional)