In a small bowl, combine the salt, pepper, chipotle chili powder, and cumin. Toss to combine.
Pat the halibut filets dry then evenly distribute the seasoning mixture over the tops of the filets and rub the mixture into the fish so that it adheres well.
Heat the oil in a cast-iron skillet (or an oven-safe skillet) over medium-high heat. When oil is very hot, but not smoking, add the halibut (skin side down) to the skillet and sear for about 3 minutes.
Carefully flip the fish over and place the skillet into the oven. Continue to cook for 8-10 more minutes, or until fish is cooked through and flakey, but not dry! (I had a 6oz piece and it took 8 minutes, my husband's was 8oz and took 10 minutes)
Remove from oven and serve immediately topped with the avocado salsa.