Slice the steak into very thin slices no thicker than 1/4 inch. Chop cress and reserve.
Cut baguette into 4 pieces and split but do not separate them so they open like a book.
Place a large skillet over medium-high heat with 2 turns of the pan of EVOO and butter. Pat sliced meat dry and season with salt and pepper. Coat the pieces in flour, shake off any excess then add to the hot pan. Grate the onion directly into the pan, cook 5-6 minutes, turning the meat occasionally.
Add stock to pan, bring up to a bubble and cook about a minute to thicken. Stir in sour cream, 1 tablespoon of lemon juice and dill. Season the sauce with pepper and remove from heat.