Bring a big pot of salted water to boil and cook tagliatelle according to the package. Reserve 1 cup pasta water, then drain and set pasta aside.
Heat olive oil in a skillet over medium-high. Add beef and brown for 4–5 minutes. Remove beef from pan.
Add onion to the same pan, cook 2–3 minutes. Add garlic, stir for 1 minute until fragrant.
Turn heat to medium. Stir in creme fraiche, Dijon mustard, and lemon juice. Simmer 2–3 minutes until slightly thickened.
Add pecorino and stir until melted. If too thick, add a splash of reserved pasta water.
Return beef to pan, toss to coat in sauce. Add tagliatelle and gently mix everything together.
Serve immediately topped with chopped parsley and extra pecorino if desired.
Notes
Sear steak on all sides for flavor. Let it rest briefly before combining with pasta to keep juicy. Add more stock or pasta water when reheating to loosen the sauce.