To prepare the watermelon salsa, in a mixing bowl, combine all the ingredients and season with salt and pepper.
To prepare spice blend, grind all spices in a mini food processor or spice grinder they are powdery. Spread the spice mixture evenly on a large plate.
Rinse the tuna fillets and pat dry. Place one side of each piece of tuna into the spice mixture. Shake off excess.
In a 10-inch nonstick skillet, heat 1 teaspoon of olive oil over high heat until almost smoking. Carefully place two of the tuna fillets, spiced side down, into the skillet. A spice crust should form within 1 minute. Using kitchen tongs, turn over the tuna, lower the heat to medium, and cook for 2 to 5 minutes to desired doneness. Repeat the process with the remaining tuna fillets. For Watermelon Salsa: