How to Make It
Place flank steak in a large zip-top plastic freezer bag. Stir together lime juice, next 2 ingredients, 1 Tbsp. cumin, 1 Tbsp. ancho chile powder, and 2 tsp. kosher salt in a small bowl. Whisk in 1/4 cup olive oil, and pour over flank steak. Seal bag, and turn to coat. Chill 1 to 12 hours.
Place oven rack about 6 inches from top of oven. Preheat oven to 450°. Rinse tomatillos, and cut into quarters. Stir together sweet potatoes, red onion, tomatillos, and remaining 1 tsp. cumin, 1 tsp. ancho chile powder, 2 tsp. salt, and 2 Tbsp. oil in a large bowl. Spread sweet potato mixture in an even layer in a heavy-duty aluminum foil-lined sheet pan.
Bake at 450° for 20 minutes. Remove from oven, and move sweet potato mixture to outer edges of pan. Place flank steak in center of pan. Increase oven temperature to broil.
Broil 6 minutes. Turn steak over, and broil 6 minutes more. (Stir vegetables if they begin to char.) Remove from oven, and let stand 5 minutes. Cut steak across the grain, and drizzle with pan drippings. Serve with sweet potato mixture, tortillas, sour cream, and fresh cilantro.