Gently insert knife tip into steak numerous times; insert garlic into cuts. Marinate steak in teriyaki sauce 10 minutes.
Cook steaks on preheated medium-hot grill 3-5 minutes per side, or until desired doneness. Let sit 5 minutes before slicing.
Heat butter in large skillet on medium-high heat; add onion and peppers, and cook 2 minutes. Add tomato and 1 Tbsp. teriyaki sauce, and cook 2-3 minutes, or until onions and peppers are tender.