Set the Instant Pot to sauté mode and heat the olive oil. Add the ground beef and sausage, breaking it up with a wooden spoon, and cook until browned, about 5-7 minutes. Drain excess fat if needed.
Add the onion, carrots, and celery to the pot. Cook for 5 minutes, stirring occasionally, until the vegetables start to soften.
Stir in the garlic and cook for 1 minute until fragrant.
Add the crushed tomatoes, tomato paste, beef broth, Italian seasoning, salt, and pepper. Stir well to combine and scrape up any browned bits from the bottom of the pot.
Cancel sauté mode, lock the lid in place, and set the valve to sealing. Cook on high pressure for 10 minutes.
Quick release the pressure, then carefully open the lid. Switch back to sauté mode and stir in the milk or cream. Let the sauce simmer for 5 minutes to thicken slightly.
Taste and adjust seasoning if needed. Serve the Bolognese over cooked spaghetti, topped with fresh basil and grated Parmesan cheese.