Preheat the oven to 450°. In a small bowl, combine the chopped thyme, rosemary and lavender leaves with 2 tablespoons of the olive oil, 1 teaspoon of salt and 1/2 teaspoon of pepper.
Carefully loosen the skin over the chicken breast and legs; don't tear the skin. Gently work the herb paste under the skin, spreading it all over the breast and legs.
Season the cavity with salt and pepper and stuff with the reserved lemon halves, garlic cloves and herb sprigs.
Set the chicken in a medium flameproof roasting pan and rub the skin with the remaining 2 tablespoons of olive oil.
Drizzle with half of the lemon juice and season generously with salt and pepper.
Roast the chicken for 20 minutes. Drizzle with the remaining lemon juice, then reduce the oven temperature to 350° and roast for about 40 minutes longer, or until the juices run clear when a thigh is pierced; baste once or twice during cooking.
Cover the bird loosely with foil if it browns too quickly. Transfer the chicken to a cutting board and let rest for 15 minutes before carving.
Spoon off the fat from the roasting pan and set the pan on 2 burners over moderately high heat.
Add the water and bring to a boil, scraping up any browned bits from the bottom of the pan. Boil the sauce until reduced to 3/4 cup, then strain and serve with the chicken.