Set grill to high. Heat oil in a small sauté pan and sauté chilies, onion, and garlic until soft. Remove from heat and set aside.
Brush tomatillos with oil and season with salt and pepper. Grill on all sides until slightly charred and soft.
Transfer tomatillo and serrano mixture to a food processor. Add lime juice, salt, and pepper and process until smooth. Add cilantro and pulse to incorporate. Hold refrigerated for service.
Set grill to high. For each serving, brush 1 portion steak with 2 teaspoons oil and rub with 1 teaspoon Cowboy Rub on both sides. Grill steak until cross-hatched and slightly charred on both sides and has reach desired temperature. Let rest 5 minutes then slice against the grain into 1/4-inch thick slices.
While steak is grilling, grill 1/4 each pepper and onion on all sides until soft. Remove from grill and slice.
Place steak, grilled peppers and onions, 1/2 cup rice, and 1/4 sliced avocado next to each other on desired plate. Drizzle steak with 1 tablespoon Memphis BBQ Sauce. Serve hot alongside 3 warmed tortillas and 2 ounces Tomatillo Salsa.
This dish could become a burrito or also be served in taco shells.
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