Turn Instant Pot on and press SAUTE function + 10 minutes. Brown sausages for 5 minutes, remove, and slice into half-inch disks, return to pot. Add olive oil, onions, salt, pepper, garlic, oregano, thyme, and red pepper flakes. Stir and cook for 2-3 minutes. Add tomato paste and stir well to incorporate.
Pour in tomatoes and chicken stock.
Turn Instant Pot off, then back on and press MANUAL + 20 minutes. Replace lid and close valve.
When the timer goes off, release the pressure manually and remove the lid. Stir in zucchini noodles and let sit for 5-10 minutes until softened to desired texture.
Portion and serve with a dollop of ricotta cheese, a sprinkle of parmesan cheese and chopped parsley if desired.