Ingredients

2 1/2 tablespoons unsalted butter, divided
1 tablespoon all-purpose flour
1 cup beef stock or broth
A 1-pound piece filet mignon, sliced in 1/4-inch thick slices
Salt and freshly ground pepper
2 tablespoons olive oil
1/2 cup thinly sliced shallots
3/4 pound cremini mushrooms, sliced or halved
3 tablespoons sour cream
1 teaspoon Dijon mustard
2 tablespoons chopped fresh dill

Directions

Melt 1 1/2 tablespoons of butter in a 1-quart heavy saucepan. Whisk in flour and cook, while whisking, for 2 minutes to make a roux. Add stock while whisking constantly, and bring to a boil. Reduce heat and simmer, whisking occasionally, for 3 minutes. Remove from heat and cover.

Season beef with salt and pepper. Heat 1 tablespoon butter and 1 tablespoon oil in a 12-inch heavy skillet. Add beef and brown on both sides but leave it pink inside. Transfer to a plate. Heat remaining 1 tablespoon each butter and oil in skillet. Add shallots and cook, stirring occasionally, until golden brown, about 3 minutes. Add mushrooms and cook until mushrooms are browned. Return meat and its juices to skillet; stir to combine. Reheat sauce over low heat; then whisk in sour cream, mustard, dill, 1/2 teaspoon salt, 1/4 teaspoon pepper. Do not let boil. Pour sauce over beef mixture and serve.