How to Make It
Open bottom vent of a charcoal grill completely. Light charcoal chimney starter filled with coconut charcoal briquettes. When briquettes are covered with gray ash, pour them onto bottom grate of grill. Adjust vents as needed to maintain an internal temperature of 300°F. Place shallot and dried red chile on unoiled grates. Grill, covered, turning occasionally, until chile is bright red and crisp, 6 to 8 minutes; remove from grill. Continue grilling shallot until very soft, about 20 minutes more. Remove from grill; let cool 10 minutes. Peel shallot; discard skins. Adjust grill vents to increase temperature to 475°F. Remove top grate, and coat with oil; place on grill.
Mash chopped lemongrass with a mortar and pestle until finely ground. Add sugar and grilled chile; mash until well combined. Add peeled shallot; mash until mixture forms a mostly smooth paste. Transfer mixture to a small bowl; stir in lime juice and fish sauce. Set aside.
Rub tuna with oil; sprinkle with salt. Place on oiled grates. Grill at 475°F, uncovered, until tuna is cooked to a depth of 1/4 inch but middle is still red, about 3 minutes per side. Transfer to a cutting board; cut into 1/4-inch-thick slices.
Place mango, rau ram leaves, lime leaves, and Fresno chile slices in a large bowl. Drizzle with 2 tablespoons lemongrass dressing, and toss to coat. Divide salad among 4 plates; top evenly with tuna slices. Drizzle about 2 teaspoons lemongrass dressing over each serving of tuna; reserve remaining dressing for another use.