Finely dice the onion into 1/4-inch pieces and thinly slice the green onions on the bias—these are your garnish and aromatics, so precise cuts matter for even cooking and presentation. Grate the cheddar cheese fresh (this melts much smoother than pre-shredded, which contains anti-caking agents). Have your tomato sauce, beef broth, pasta, mustard, and relish measured and ready to go. Heat the olive oil in a large pot or Dutch oven over medium-high heat until shimmering, then add the diced onion and ground beef. Cook, breaking up the beef with a spoon, for 5-7 minutes until the beef is deeply browned and the onion is softened and beginning to caramelize—this fond (browned bits) will build incredible flavor in your sauce.

Carefully drain off excess fat from the pot, leaving about 1 tablespoon to coat the bottom—this keeps your sauce from being greasy but maintains richness. Sprinkle the flour over the browned beef and onion mixture, then stir constantly for 1 minute to create a roux, coating all the meat and cooking out the raw flour taste. This roux will thicken your final dish and give it body. Pour in the tomato sauce and beef broth, stirring well to incorporate any browned bits from the bottom of the pot and to dissolve the roux evenly.

Add the dry pasta directly into the pot and stir to submerge it in the liquid as much as possible. Bring the mixture to a boil over medium-high heat, then reduce to medium and cover with a lid. I like to stir occasionally during the first few minutes to prevent sticking and ensure even cooking. Simmer for about 10-12 minutes (check your pasta box for cooking time and aim for the lower end, as it will continue cooking slightly off heat) until the pasta is tender and most of the liquid has been absorbed—you should have a creamy, cohesive dish, not soupy.

Remove from heat and stir in the mustard and relish for brightness and tang that mimics a classic cheeseburger's condiment profile. Add the freshly grated cheddar cheese in handfuls, stirring gently after each addition until it's completely melted and the dish becomes creamy and cohesive. I find that adding cheese off-heat (rather than over heat) prevents it from becoming grainy or separating.

Divide the cheeseburger pasta into bowls or plates and top each serving with the sliced green onions for color, freshness, and a mild onion bite. Serve immediately while hot and creamy.