Get a large pot of water boiling for your pasta. Don't forget to salt it – it should taste like the sea!
While that's heating up, slice your mushrooms, thinly slice those onions, and mince your garlic if using.
Cut your leftover roast beef into bite-sized pieces. If there's a bit of fat, don't stress about trimming every last bit – that's flavor!
Mix 1 tablespoon cornstarch with 2 tablespoons cold water in a small bowl until smooth.
Measure everything else out so it's ready to grab when you need it.
Cook the pasta: Toss your rigatoni into that boiling water and cook until it's just shy of al dente – about 2 minutes less than the package says.
Before you drain it, scoop out ¼ cup of that starchy pasta water.
Heat up a large, deep skillet over medium heat. Give it a good spray with that low-calorie cooking spray.
Add your onions and let them cook for 3-4 minutes.
Add the mushrooms (and garlic if you're using it). Cook for 5-7 minutes.
Pour in that beef broth and Worcestershire sauce. Let it gently simmer and reduce for 2-3 minutes.
Stir in the Dijon mustard, then pour in your cornstarch slurry while stirring constantly.
Turn the heat down to low. Add your quark and sour cream, stirring gently until smooth.
Gently stir in your leftover roast beef pieces.
Add your drained rigatoni directly to the sauce, folding everything together.
If your skillet can go in the oven, great! If not, transfer everything to a baking dish.
Sprinkle that Parmesan cheese evenly across the top.
Place under a medium-high broiler for about 5 minutes.
Let it sit for 2-3 minutes before serving.
Sprinkle with fresh chopped parsley just before bringing it to the table.
