Preheat broiler. In a small bowl, mix thyme, salt, pepper, balsamic vinegar and rind of one lemon. Rub on both sides of steak. Place on a broiler safe pan, and broil for 10 minutes per side. In a small saucepan, melt butter on medium-low heat, not browning. Add onions, green peppers and soften for 5 minutes. Add flour and mix very well. Add beef broth, salt and pepper and bring to a boil mixing constantly. Reduce to medium heat, reduce gravy about half, approximately 15 minutes. Check for salt and pepper, adding if needed. Remove from oven and let meat rest for 5 minutes. Thinly slice steak and drizzle with gravy -- enjoy!