Lightly beat an egg in a large bowl. Mince garlic cloves and grate Parmesan cheese. Add both to the bowl. Add breadcrumbs, milk, kosher salt, and Italian seasoning. Add the ground beef and sausage to the bowl.
Gently mix with your hands until well combined. Form and shape into 12 meatballs (about 2 tablespoons each). Set aside.
Add 1/2 cup of water and sauce to the pot. Stir to combine. Add meatballs to the bottom without stacking them and with minimal touching.
Break the spaghetti in half and spread them in two layers over the meatballs. Do not stir.
Pour the remaining 1 1/2 cups of water over the pasta. Remember not to stir.
Seal the Instant Pot. Set to cook on HIGH pressure for 8 minutes.
Open the pressure release valve (quick release) as soon as the 8 minutes are up. Open the pressure cooker and stir the spaghetti into the sauce. Serve with grated Parmesan cheese.