Grilled Tuna Sandwich with Lemon-Chili Mayo

• Preheat grill to medium-high. • Lightly brush bread slices with oil. Brush tuna with the remaining oil and sprinkle with ½ teaspoon chili powder and salt. • Grill the tuna, turning once, until cooked through, 4 to 6 minutes per side. Transfer to a clean cutting board. Grill the bread, turning once, until beginning to brown, about 1 minute per side. • Combine mayonnaise, lemon zest, lemon juice and the remaining 1 tablespoon chili powder in a small bowl. Thinly slice the tuna with the grain. • To assemble the sandwiches, spread about 2 tablespoons of the lemon-chili mayonnaise over 4 slices of bread. Top with about ¾ cup watercress (or arugula), the tuna and the remaining slices of bread. • Make Ahead Tip: Wrap and refrigerate the sandwiches for up to 4 hours.