In a large heavy skillet cook onions in 2 tablespoons oil over moderate heat, stirring, until golden and add flour. Cook mixture over moderately low heat, stirring, 2 minutes and stir in wine, tomato paste, broth and gravy. Simmer sauce, stirring, 3 minutes and stir in mustard.
Preheat oven to 400 degrees. Spoon half of sauce into 2-quart shallow flameproof baking dish and arrange beef on it, overlapping slices. Spread remaining sauce over beef.
In a small bowl toss bread crumbs with remaining 1 1/2 tablespoons oil and sprinkle over sauce. Bake miroton in upper third of oven until just bubbling around edges, about 5 to 10 minutes. If crumbs are not golden, put miroton under broiler 1 to 2 minutes.