How to Make It
Preheat grill or grill pan to medium-high (about 450°F). Combine lemon zest, lemon juice, rosemary, salt, pepper, and 1 tablespoon oil in a large ziplock bag. Add chicken to bag and seal. Turn bag to coat chicken with marinade. Set aside at room temperature.
Brush leeks with remaining 1 tablespoon oil. Place leeks, cut sides down, on grill grate; grill, uncovered, until grill marks form, 6 to 7 minutes. Transfer leeks to a large piece of aluminum foil. Sprinkle with butter and garlic; fold up edges of foil to seal.
Place chicken, lemon halves, and foil packet on grill grates. Grill chicken until cooked through and grill marks form, 2 to 3 minutes per side. To serve, squeeze lemon over chicken and leeks. Garnish with rosemary.