Flank Steak in Mushroom Sauce

Flank steak is a lean, tough cut that turns silky and fork-tender after 6 to 8 hours in a slow cooker. The braising liquid here does serious work: sauterne wine, beef broth, soy sauce, garlic, ketchup, and mustard combine into a sauce that’s savory, slightly sweet, and deeply beefy.

That combination of ketchup and mustard in the braising liquid might raise an eyebrow, but it’s a classic slow cooker trick. The ketchup adds sweetness and body, the mustard brings a sharp bite that cuts through the richness. You won’t taste either one distinctly in the finished dish. They just round out the sauce.

The mushrooms go in at the very end, during the final 20 to 30 minutes on high. This keeps them tender with some bite instead of dissolving into mush after hours of cooking. The cornstarch slurry added at the same time thickens the braising liquid into a glossy gravy that clings to every slice.

Kitchen Tips

• Don’t trim the flank steak. Any fat renders out during the long braise and adds flavor to the sauce.
• Slice against the grain when serving. Flank steak has very visible muscle fibers. Cutting across them is the difference between tender slices and chewy strings.
• If the sauce is thinner than you’d like after adding cornstarch, leave the lid off for the final 10 minutes on high to let it reduce.
• Serve over egg noodles, mashed potatoes, or rice to soak up the gravy.

Variations

• Use a dry red wine like Cabernet instead of sauterne for a darker, more robust sauce.
• Add a handful of pearl onions with the mushrooms for extra sweetness.