Mix the spice blend. Combine the smoked paprika, garlic powder, onion powder, thyme, cayenne, salt, and black pepper in a small bowl.
Coat the tuna. Brush both sides of each tuna steak with melted butter, then press firmly into the spice blend, coating both sides completely.
Get the pan properly hot. Heat a cast-iron skillet over high heat for 3-4 minutes, until it's genuinely smoking.
Sear the tuna. Add the olive oil to the hot pan, then sear the tuna for 90 seconds to 2 minutes per side. Transfer to a plate and let it rest.
Cook the snap peas. In the same skillet, add the snap peas and garlic. Toss constantly for 2-3 minutes, until bright green and tender-crisp.
Serve. Slice the tuna against the grain, plate alongside the snap peas, and serve with lemon wedges.