Pat the tilapia fillets completely dry on both sides with paper towels. This step matters. Any surface moisture will steam the fish instead of searing it, and you will not get that dark spice crust. Set the fillets aside on a clean plate.

In a small bowl, combine the smoked paprika, cayenne, garlic powder, onion powder, cumin, oregano, 3/4 teaspoon salt, and black pepper. Stir until evenly mixed. Press this spice blend firmly onto both sides of each tilapia fillet, covering the entire surface. You want a visible, even coating.

Make the salsa while the spice sits on the fish. In a medium bowl, combine the diced avocado, cucumber, red onion, cilantro, lime juice, and 1/4 teaspoon salt. Stir gently so you do not mash the avocado. Taste and add more lime or salt if needed. Set aside at room temperature.

Place your cast iron skillet over medium-high heat and let it heat for a full 2 to 3 minutes before adding any oil. The pan should feel very hot when you hold your hand 3 inches above the surface. Add the oil and swirl to coat. You are ready to cook when the oil shimmers and just begins to show a faint wisp of smoke.

Lay the seasoned fillets in the hot skillet, leaving space between them. Cook without moving them for 3 to 4 minutes. You will see the edge of the fish turn opaque as it cooks upward. When the bottom two-thirds of the fillet looks white and only the top center still appears translucent, it is time to flip.

Flip each fillet carefully using a wide fish spatula. Cook for another 2 to 3 minutes on the second side. The fish is done when it flakes easily when pressed gently with a fork in the thickest part. Transfer to plates immediately so the residual heat of the skillet does not overcook the fish.

Spoon the cucumber avocado salsa generously over each fillet. Garnish with extra cilantro and a lime wedge on the side. Serve right away while the crust is still crisp and the salsa is cool and fresh.