Halve steak lengthwise, then cut crosswise into 1/4-inch-thick strips.

Cook vegetables with 3 tablespoons water, covered, in a large nonstick skillet over medium-high heat until crisp-tender, about 4 minutes. Transfer vegetables to a bowl; drain any excess water. Set aside.

Heat same skillet over medium-high heat until hot. Add 1/2 the steak and 1/2 the garlic; cook until outside surface of steak is no longer pink, 1 to 2 minutes. Transfer to a plate. Repeat with remaining steak and garlic.

Return steak and vegetables to skillet. Add stir-fry sauce and red pepper flakes; heat through. Serve over hot rice; sprinkle with peanuts.