Brown the beef. Set Instant Pot to Sauté – High. Add olive oil, then ground beef and diced onion. Break up the beef with a wooden spoon and cook 4–5 minutes until the beef is no longer pink and the onion is translucent. Drain excess fat if needed, leaving about a tablespoon for flavor.
Bloom the seasonings. Add the minced garlic, tomato paste, Worcestershire sauce, smoked paprika, onion powder, kosher salt, black pepper, and mustard powder. Stir constantly for 60–90 seconds until the tomato paste darkens and becomes fragrant. This step deepens the flavor — don't skip it.
Deglaze the pot. Pour in ½ cup of the beef broth. Using a wooden spoon, scrape the entire bottom of the pot clean — corners included. The bottom should be smooth with no stuck-on bits. This is the most important step for preventing the burn notice.
Layer the pasta. Pour in the remaining 3½ cups of broth. Add the dry elbow macaroni and gently press it down with the back of your spoon until submerged. Do not stir. Stirring pulls pasta down to the heating element and causes scorching.
Pressure cook. Cancel sauté. Lock the lid and set the valve to Sealing. Press Pressure Cook (or Manual) on High Pressure for 3 minutes. The Instant Pot will take 8–10 minutes to come to pressure before the cook time begins.
Quick release. When the cook time ends, carefully turn the valve to Venting for a quick release. Wait for the float pin to drop, then unlock and open the lid. The pasta will look soupy — that's correct.
Add the dairy. Stir in the milk. If using cream cheese, add the cubes now and stir until melted. Add the shredded cheddar in three handfuls, stirring between each addition, until the sauce is smooth and creamy.
Rest and serve. Let the dish sit uncovered for 2–3 minutes. The sauce will thicken visibly as it cools slightly. Taste and adjust salt and pepper. Garnish with fresh parsley and serve immediately.