For the steak: In a small bowl, mix together the chili powder, salt and pepper. Sprinkle it all over the London broil and rub. Wrap tightly with plastic wrap and rest for 2 hours.
For the salad: In a large bowl, add the tomatoes, parsley, fennel bulb and fronds and garlic. In a small bowl, whisk together the agave nectar, red wine vinegar, olive oil, a pinch of salt and a few grinds of black pepper. Pour this mixture over the vegetables and toss. Rest at room temperature, tossing halfway through, for about 2 hours.
Preheat a grill to high heat and leave one side of the grill off for indirect heat. Unwrap the steak and drizzle it with olive oil. Place the steak over the direct heat of the grill and cook for 5 to 6 minutes. Flip the steak, moving it to the indirect heat side, close the grill and cook until the internal temperature registers 130 degrees F for medium rare. Remove from the grill, cover gently with aluminum foil and rest for 10 minutes.
Slice the meat against the grain, top with the tomato salad and serve.