Preheat your grill or stovetop grill pan over medium-high heat.
In a small bowl, mix olive oil, garlic powder, onion powder, salt, and pepper. Rub this mixture all over the sirloin steak.
Grill the steak for about 4-5 minutes on each side for medium-rare, or until it reaches your desired doneness. Remove from the grill and let it rest for 5 minutes before slicing.
While the steak is resting, heat a skillet over medium heat and add the sliced bell peppers. Sauté for about 5-7 minutes until they are tender and slightly caramelized.
Spread a generous layer of pesto on each piece of flatbread.
Top the pesto with sliced steak, sautéed bell peppers, and shredded mozzarella cheese.
Place the flatbreads on the grill or in the oven at 400°F for about 5 minutes, or until the cheese is melted and bubbly.
Remove from heat, garnish with fresh basil leaves if desired, and slice into pieces.