In a bowl (do not use aluminum) mix the chillies, half of the coriander, garlic, paprika and pepper together. Stir in the olive oil and lemon juice. Place the tuna in the marinade and rub it in thoroughly. Cover and let stand in the refrigerator for 2 hours.
Light a charcoal fire in a grill or preheat the grill.
Remove the fish from the marinade, season with salt, place on a grate brushed with oil or in a grill pan and grill on the charcoal grill or in the oven on both sides - depending on the degree of doneness you want, about 3 minutes per side for medium or longer.
Arrange the tuna on a preheated platter or on plates, garnish with the remaining coriander leaves and serve immediately.