Brown the beef. Cook the ground beef in a large skillet over medium-high heat, breaking it up as it browns, about 6–7 minutes. Drain excess fat, leaving a thin layer for flavor.
Cook the mushrooms and onion. Add the mushrooms and onion to the pan with the beef and cook for 5–6 minutes, until the mushrooms have released their liquid and started to brown and the onion is softened.
Add garlic and flour. Stir in the garlic and cook for 30 seconds until fragrant. Sprinkle the flour over everything and stir constantly for about 1 minute, until it’s no longer visible as a raw powder.
Build the sauce and cook the noodles. Pour in the beef broth, Worcestershire sauce, and Dijon mustard, stirring to work out any flour lumps. Add the egg noodles directly to the pan, bring to a simmer, then cover and cook for 8–10 minutes, stirring occasionally, until the noodles are tender and the liquid has reduced into a thickened sauce.
Take the pan off the heat before adding sour cream. Remove the skillet from the heat entirely not just turn the burner down and let it sit for about 30 seconds before stirring in the sour cream. Pro tip: stir the sour cream in gradually, working it into the sauce in two or three additions rather than dumping it all in at once, so it has an easier time incorporating smoothly. Adding it off the heat instead of into an actively simmering pan is what keeps the proteins in the sour cream from separating and turning the sauce grainy.
Let the dish rest before serving. Let the stroganoff sit, uncovered, for about 5 minutes once the sour cream is fully incorporated. This short rest matters more than it looks like it should: the sauce is still slightly thin right when the sour cream goes in, but the noodles keep absorbing liquid and the starch already in the sauce keeps setting up as everything cools slightly, so a sauce that seems a bit loose at first will noticeably thicken to the right coating consistency by the time you serve it.
Season and serve. Taste and adjust salt and pepper the broth and Worcestershire already bring a fair amount of salt, so season gradually. Top with fresh parsley and serve warm.
