Preheat grill to medium-high heat and oil the grates or heat a grill pan over medium-high heat.
Grill the top sirloin steak, covered if using an outdoor grill, until it reaches desired doneness, about 11–15 minutes turning occasionally (for preheated gas grill expect 13–16 minutes).
While the steak cooks, make the dressing: whisk together agave (or honey), peanut butter, hot water, soy sauce, rice vinegar, and sesame oil in a bowl until smooth and well combined.
Arrange the prewashed greens on a serving platter, then top with sliced mango, sliced avocado, shredded carrot, and diced bell pepper.
Remove the steak from the grill and let it rest 5 minutes, then carve into thin slices and lightly season with coarse sea salt.
Arrange the sliced steak over the salad, drizzle with the peanut-agave dressing, and sprinkle with chopped dry roasted peanuts if using.
Serve immediately with lime wedges to squeeze over the salad as desired.