Lightly beat egg and milk together in a large bowl. Mix in bread crumbs, adding up to an additional 1 tablespoon milk if mixture looks dry. Mix in ground beef, pork, onion, Parmesan cheese, garlic, Italian seasoning, salt, pepper, and red pepper flakes. Form mixture into five 3 1/2- to 4-ounce meatballs.

Turn on a multi-functional pressure cooker (such as Instant Pot®) and select Saute function. Add butter and oil and heat until butter has melted. Add meatballs and cook until browned, 1 to 2 minutes per side. Add pasta sauce and hit Cancel.

Close and lock the lid. Select high pressure according to manufacturer's instructions; set timer for 8 minutes. Allow 10 to 15 minutes for pressure to build.

Release pressure using the natural-release method according to manufacturer's instructions, about 10 minutes. Then switch to the quick-release method according to manufacturer's instructions, about 5 minutes. Unlock and remove the lid.

Meanwhile, bring a large pot of lightly salted water to a boil. Cook spaghetti in the boiling water, stirring occasionally, until tender yet firm to the bite, about 12 minutes. Drain.

Place cooked spaghetti into 5 individual pasta bowls. Top each serving with sauce and a giant meatball.