Pat steak dry; sprinkle with 1/2 tsp. each salt and pepper. Heat oil in a large frying pan over medium-high heat. Cook steak, turning once, until browned but still pink in center, about 3 minutes total. Arrange steak on plates.
Add butter, mushrooms, garlic, and shallots to pan and cook until starting to brown, about 2 minutes. Add broth and remaining 1/2 tsp. each salt and pepper. Simmer until most of liquid evaporates, about 3 minutes.
Spoon mushrooms over steak. Dollop each piece of steak with about 1/8 tsp. mustard and sprinkle plates with watercress.
*Find clamshell mushrooms in well-stocked grocery stores.
Note: Nutritional analysis is per serving.