Chop watercress and reserve.
Cut baguette into 4 pieces and split but do not separate.
Heat a large skillet over medium to medium high heat with 2 tablespoons extra-virgin olive oil and butter.
Pat sliced meat dry. Season the meat with salt and pepper and coat in flour. Shake off excess then add to very hot pan. Grate the onion directly into the meat. Cook meat 5 to 6 minutes, turning occasionally. Add stock to pan and cook to thicken 1 minute, stir in sour cream, a tablespoon of lemon juice and dill. Season the sauce with pepper. Remove from heat. Pile meat into cut baguette and top with cress.