"You won't miss the noodles one bit...."
medium spaghetti squash, halved and seeds removed
extra-virgin olive oil, divided
Kosher salt
Freshly ground black pepper
dried oregano
baby spinach
ricotta (preferably whole milk)
medium onion, chopped
cloves garlic, minced
ground beef
(32-oz.) jar marinara
torn basil, plus more for garnish
shredded mozzarella
Grated Parmesan, for garnish (optional)