Wrap the tuna in plastic wrap and place in the freezer for an hour or so to firm for easier cutting.
Place a large skillet over medium heat and scatter the garlic on one side. When the pan is hot, toast the chiles one by one: use a metal spatula to press the pieces flat against the hot surface until they blister and release their aroma, about 10 seconds per side. (When properly toasted, the inside of each chile piece will have lightened in color, but not developed any dark spots.) Transfer to a bowl, cover with hot tap water and weight with a plate to keep submerged. Meanwhile, turn the garlic occasionally until it is completely softened (it will blacken in spots)—it will take 10 to 15 minutes. Cool until handleable, then peel.
When the chiles have soaked 30 minutes, drain and place in a blender jar with the garlic and lime juice. Blend to a smooth puree, adding a little water if necessary to keep everything moving through the blades. Press through a medium-mesh strainer into a bowl. Season with salt, usually about 1 teaspoon. Refrigerate until you’re ready to serve.
With a sharp knife, cut the tuna into small dice—I like ¼-inch cubes—and scoop into a bowl. When you’re ready to serve, scoop the onion into a strainer, rinse under cold water, shake off the excess and add to the tuna. Mix in the red-chile seasoning and chopped cilantro. Serve—whether on a crispy base like a chip, molded on a plate or in a small bowl or glass—decorated with cilantro leaves.