How to Make It
Heat 1 tablespoon oil in a large skillet over high. Pat steak dry; sprinkle with 1/2 teaspoon salt and 1/2 teaspoon pepper. Cook until browned and a thermometer inserted in thickest portion registers 135°F, 4 to 5 minutes per side. Remove from skillet; cover to keep warm. Wipe skillet clean.
Heat skillet over medium-high. Add broccoli, onion, and remaining 1 tablespoon oil. Cook, stirring occasionally, until broccoli is slightly charred, about 4 minutes. Add 1/4 cup water and garlic; cook until water evaporates and broccoli is crisp-tender, about 2 minutes. Sprinkle with remaining 1/4 teaspoon salt.
Stir together mayonnaise, lemon juice, and remaining 1/4 teaspoon pepper in a small bowl. Thinly slice steak against the grain. Serve steak with broccoli and mayonnaise mixture.