Bring a large pot of water to a boil, add a generous few pinches of salt and cook pasta shells according to package instructions, about 11 minutes. Drain and set aside.
Meanwhile, heat olive oil in large skillet over medium-high heat. Add ground beef and brown, stir and crumble about 4 or 5 minutes. Add onion to the beef and cook another 2 to 3 minutes until onions are translucent.
Stir in garlic, Italian seasoning, dry mustard, crushed red pepper, salt and Worcestershire and continue cooking until fragrant, about 1 minute.
Sprinkle with flour, stir to incorporate and cook off the flour taste, 1 to 2 minutes.
Gradually whisk in the beef broth and crushed tomatoes, bring to a boil. Reduce heat to a simmer and stir occasionally until slightly reduced and starting to thicken, about 7 or 8 minutes.
Add the cooked pasta shells to the skillet, stir in heavy cream and sour cream and heat through, about 2 minutes. Season with kosher salt and pepper.
Add the grated cheese and stir to combine and melt, about 2 more minutes.
Serve immediately, or let rest 5 minutes or so off heat to thicken more. Sprinkle with grated parmesan, if desired.