Begin by preheating the grill to a medium-high heat.
While the grill is heating up, in a bowl, combine the diced avocado, red onion, bell pepper, lime juice, and cilantro.
Mix them gently and season the relish with salt, pepper, and chili flakes. Set this avocado relish aside.
Season the tuna steaks with salt, pepper, and a drizzle of olive oil.
Once the grill is ready, place the tuna steaks on it. Grill each side for about 3-4 minutes, ensuring grill marks appear and the center remains slightly pink.
Once grilled to perfection, remove the tuna steaks and place them on serving plates.
Generously top each steak with the prepared avocado relish.