Slice the sirloin steak into thin strips, about ½ inch wide.
In a bowl, whisk together brown sugar, garlic, cumin, chili powder, kosher salt, pepper, orange juice and olive oil.
Place the steak and marinade in a zip to bag and press out the excess air. Turn the bag over a few times to coat the steak. Refrigerate at least one hour or up to 8 hours.
Prepare a grill fire to 350°.
Drizzle onions and peppers with olive oil and season with salt and pepper.
Place onions and peppers onto the grill, using a grill pan if your grates are large.
Cook for 5 minutes with the grill closed.
Remove steak from marinade and place on the grill.
Cook with the grill closed about 8 minutes.
Stir the steak, peppers and onions and continue to cook another 5 minutes or until steak reaches 140° using a digital meat thermometer.
Remove steak, peppers and onions to a platter or rimmed sheet pan. Cover with foil and set aside until ready to serve.