Preheat oven to 350 degrees. Bring a large pot of water to a rolling boil.
Heat a large over-proof skillet to a medium heat. Add ground beef. Use a wooden spoon to break the meat up while it cooks. Once it's almost all the way cooked though and starting to brown, add the onions, garlic, mushrooms and salt. Continue to cook until mushrooms and onion are softened and beef is cooked through.
Add flour, stir to combine. Pour in beef stock, worcestershire sauce, dijon and remaining 1/4 tsp salt. Stir to combine. Bring to a boil and reduce to a simmer. Simmer until thickened, about 5 minutes. Turn the heat on low and add Greek yogurt. Stir until combined. Season the sauce to taste with salt and pepper.
While the sauce thickens, drop the pasta in the boilling water. Cook until just under aldente. Reserve pasta water. Use a slotted spoon or strainer to transfer the pasta to the sauce. Toss until combined. If needed, add a little bit of pasta water to loosen the sauce up. Smooth out the noodles and cover with mozzarella cheese. Pop in the oven until the cheese has melted and the pasta is hot.