Preheat oven to 425°F; line a sheet pan with foil.
Mix lemon juice, zest, 2 tablespoons oil, thyme, rosemary, garlic, salt, and pepper in a small bowl.
Pat chicken dry; rub marinade all over, including under skin.
Toss peppers and onion with remaining 1 tablespoon oil, salt, and pepper.
Arrange chicken skin-side up on pan; scatter vegetables around it and tuck lemon slices among vegetables.
Roast 35-40 minutes until chicken reaches 165°F internally and skin is crisp and golden.
Rest 5 minutes; squeeze roasted lemon over everything and serve.