Chop the pancetta into minced pieces as best you can. Set aside. Mince the white onion, celery stalks, and carrots. You should have about 3/4 cup each of the celery and carrots after mincing but doesn't have to be exact.
Press saute function on the pressure cooker and adjust to low heat. Add the pancetta and cook until the fat is rendered, about 3-4 minutes.
Add butter, onion, celery and carrots. Cook until the onions are translucent, about 5-6 minutes.
Add ground beef and season with some salt and pepper to taste. Saute until barely browned, about 4 minutes. While cooking the meat, break it up with a wooden spoon as it cooks.
Add the wine or grape juice and cook until it reduces down, about 3-4 minutes.
Add crushed tomatoes, bay leaves, 1/2 teaspoon salt and fresh black pepper. Cover, and cook on high pressure for 15 minutes.
When time is up, let pressure release naturally for about 10 minutes, then switch to Vent and finish releasing pressure. Remove lid, take out bay leaves, stir in the grated Parmesan (this helps thicken the sauce), half & half. If at this point you feel the sauce still isn't as thick as you'd like, you can turn on the Saute function again at low heat and cook the Bolognese down a bit more before serving, about 5-10 minutes.
Serve over your favorite pasta, garnished with shaved or shredded Parmesan and fresh basil leaves.