1. Marinate steak: Peel garlic; peel and slice ginger. Place both in a blender with peanut butter, 1/4 cup soy sauce, 1/4 cup vinegar, sugar, red-pepper flakes, and 2 tablespoons water. Blend until smooth. Place steak in a resealable plastic bag or shallow dish; add all but 1/2 cup sauce and refrigerate. Cover and store reserved sauce at room temperature.

2. Prep broccoli: Cut florets from broccoli (reserve stalks for another use); cover and refrigerate.

1. Broil steak: Heat broiler, with rack set 4 inches from heat. Lift steak from marinade; place on a broilerproof baking sheet. Broil until medium-rare, 8 to 10 minutes. Transfer to a large plate, and cover loosely with aluminum foil; let rest, 5 to 10 minutes.

2. Cook broccoli: While steak is broiling, thinly slice scallions, keeping whites and greens separate. In a large skillet, heat oil over medium. Add broccoli, scallion whites, and 1/4 cup water; season with salt. Cover skillet, and cook, tossing occasionally, until broccoli is just tender, 8 to 10 minutes. Uncover; add remaining tablespoon soy sauce and tablespoon vinegar. Cook, tossing, until liquid is evaporated, 1 to 2 minutes. Mix in scallion greens.

3. To serve, thinly slice steak across the grain; serve with broccoli and reserved peanut sauce.