Press sauté mode on the instant pot, add oil and wait 3 minutes.
Once the oil is hot, add the ground beef and sauté, using a spoon to break it into small pieces. Cook the meat long enough for it to be fully browned.
Add onion and bell pepper and cook for another 5 minutes, or until the onion is softened.
Add the beans, tomatoes, chili powder, cumin, salt and pepper, pasta, and one cup of cheddar cheese. Give it a stir and make sure the pasta is covered by liquid.
Close the lid, and place the valve in sealing mode. Select high pressure and cook for 5 minutes. (Keep in mind that it will take about 20 minutes for the pressure to rise and start cooking.) If you're using smaller macaroni noodles, you only need to cook for 3-4 minutes.
Once cooked, release the pressure immediately, open the lid and stir it. Add the remaining cheese and milk (optional), and stir until melted.
Serve hot in a bowl; add more cheese and green onion on top.