1. Preheat the broiler. Cut a 1-lb. skirt steak, trimmed of fat, crosswise into thirds, season with tsp. each kosher salt and pepper, and sprinkle with 1 Tbs. fajita spice rub on all sides. Spread 1 thinly sliced red onion, 1 thinly sliced red sweet pepper and 1 thinly sliced yellow sweet pepper in a large foil-lined 15x10x1-inch baking pan. Toss with 1 Tbs. olive oil, 1 tsp. fajita spice rub, and 1/4 tsp. each salt and pepper. Arrange the steak pieces over the vegetables. Broil 4 to 5 inches from the heat until the steak is browned on top, about 5 to 6 minutes. Flip the steak pieces over, stir the veggies and broil again until the steak is lightly browned and cooked to your liking (140 degrees F for medium), about 3 to 5 minutes longer.
2. Transfer the steak to a cutting board and let rest. Whisk 1/2 cup sour cream with 1/2 tsp. lime zest, 1 tsp. lime juice, and 1 Tbs. water. Slice the skirt steak thinly against the grain. Serve it with the broiled vegetables over 2 cups cooked whole grains (such as quinoa) divided among 4 bowls. Garnish with shredded cheddar cheese, avocado slices and the lime crema. Serve with hot sauce, if desired.