Cut beef steak lengthwise in half, then crosswise into 1/4-inch-thick strips.
Combine vegetables and 3 tablespoons water in a large nonstick skillet. Cover and cook over medium-high heat until crisp-tender, about 4 minutes. Remove vegetables and drain.
Heat same pan over medium-high heat until hot. Add 1/2 of the beef and 1/2 of the garlic; cook until the outside surface of beef is no longer pink, 1 to 2 minutes. Remove. Repeat with remaining beef and garlic.
Return all beef and vegetables to the pan. Add stir-fry sauce and crushed red pepper; heat through. Serve over hot rice. Sprinkle with peanuts, if desired.