Prepare a medium (375°F) gas or charcoal grill fire.
Meanwhile, in a small bowl, mix the ancho powder with 1 teaspoons of the sugar, 2 teaspoons salt, and 3/4 teaspoons pepper. Pat the steak dry, sprinkle the ancho mixture on both sides, and let sit at room temperature while the grill heats.
Set the onion rounds and jalapenos on a large plate and sprinkle with 1 Tbs. of the oil, 1/2 teaspoons salt, and 1/2 teaspoons pepper.
In a large bowl, toss the cabbage with the lime juice, the remaining 1 teaspoons sugar, and 1/2 teaspoons salt. Add the remaining 1 Tbs. oil and toss again.
Put the steak, onions, and jalapenos on the grill. Grill the steak, flipping once, until cooked to your liking, 5 to 6 minutes for medium rare (130°F to 135°F). Grill the vegetables, flipping once, until grill marks appear and they’re just tender, 4 to 6 minutes for the jalapenos and about 8 minutes for the onion. (Move the onions to a cooler part of the grill or lower the heat if they start to burn.) Transfer the steak and vegetables to a cutting board; let the steak rest for 5 minutes.
Chop the onions and finely chop the jalapenos, add to the cabbage mixture, and toss well. Season to taste with salt, pepper, and lime juice.
Spread 4 tortillas on the grill and heat, flipping once, until warm and softened, about 1 minute. Stack the warmed tortillas in a kitchen towel; repeat with the remaining tortillas.
Thinly slice the steak and put a few slices on each of the warm tortillas. Top with the slaw and a dollop of sour cream. Serve with the lime wedges.