Mix the spice rub. In a small bowl, combine smoked paprika, chili powder, cumin, garlic powder, onion powder, oregano, cayenne, brown sugar (if using), salt, and pepper.
Prep the salmon. Pat the fillets dry with paper towels. Rub with olive oil on all sides. Coat generously with the spice mixture, pressing it in so it adheres.
Make the salsa. In a medium bowl, gently toss avocado, tomatoes, red onion, jalapeño, and cilantro.
Add lime juice, olive oil, salt, and pepper. Taste and adjust lime or salt. Set aside.
Heat the pan. Place a large cast-iron or heavy skillet over medium-high heat.
Add a thin layer of oil and let it shimmer. You want it hot before the fish goes in.
Sear the salmon. Place fillets in the skillet, presentation side down. Cook undisturbed for 3–4 minutes until a dark crust forms and the fish releases easily.
Flip and cook another 2–4 minutes, depending on thickness, until just opaque in the center.
Check doneness. For the most tender result, aim for an internal temperature of 125–130°F in the thickest part. The flesh should flake but still be moist.
Finish and serve. Rest the salmon for 2–3 minutes. Spoon avocado salsa over the top.
Serve with lime wedges and your favorite sides.