Preheat a smoker to 225 degrees F (110 degrees C) according to manufacturer's instructions.
Mix brown sugar and 1/4 cup salt together in a bowl.
Score ahi tuna skin with a sharp knife; rub with brown sugar and salt mixture.
Smoke tuna until an instant-read thermometer inserted into the center reads 140 degrees F to 155 degrees F (60 degrees C to 68 degrees C), about 1 hour.
Mix masa corn flour and water together in a bowl until a slightly sticky dough forms. Divide dough into large balls.
Heat a lightly oiled griddle over medium-high heat. Flatten balls of masa to 1/4-inch thickness and cook until golden brown, 3 to 4 minutes per side. Cool sopes on a wire rack.
Preheat a grill for medium-high heat and lightly oil the grate.
Place tomatillos, half the onion, and 1 jalapeno pepper on the grill; cook until charred on all sides, about 5 minutes. Transfer to a blender; blend into a smooth sauce. Season with salt.
Dice remaining onion and jalapeno. Mix with tomatoes, avocados, and cilantro in a bowl. Season with lime juice and salt.
Place cooked sopes on serving plates. Top with smoked tuna, tomatillo sauce, and avocado mixture. Garnish with shredded cabbage and queso fresco.
