1.Preheat oven to 250°F
2.Sear steak, seasoned with salt and pepper, in skillet with 1 T hot olive oil, 2 min per side until browned.
3.Place on oven-safe dish in oven with temperature probe until desired temperature is reached (130°F rare).
4.Rest 5 min, slice, thinly.
5.While steak is finishing, melt butter in pan used to sear steak and sauté onions until soft.
6.Add mushrooms and cook until golden.
7.Add flour, cook, stirring until well mixed.
8.Slowly add stock with constant stirring.
9.Add mustard and crème fraîche, stir.
10.Simmer until thickened, about 5 min.
11.Pour over sliced beef, serve