1. Combine mushrooms, broth, soy sauce, ginger and garlic in slow cooker. Pat roast dry and sprinkle with salt and pepper.
2. Warm oil in a large skillet over medium-high heat. Brown beef on all sides, about 8 minutes total. Transfer to slow cooker; turn to coat with soy sauce mixture. Cover and cook on low until tender, 6 to 8 hours.
3. Transfer roast to cutting board, tent with foil and let rest for 15 minutes. Whisk cornstarch and 1/2 cup liquid from slow cooker and return to cooker, whisking until sauce thickens, 1 to 2 minutes. Whisk in vinegar. If sauce is too thick, whisk in 1 or 2 additional Tbsp. of broth. Slice meat. Serve with sauce, topped with scallions, if desired.