This recipe is one of the most popular dishes prepared by chef Cindy Pawlcyn at her Mustards Grill restaurant in Napa Valley.
Prepare the vinaigrette: Pull the heads of garlic apart, peel the cloves, and remove any green sprouts. Place the garlic in a small saucepan. Using a paring knife, remove the peel from the lemons. Carefully remove the pith from the peels and add the peels to the saucepan; discard the pith. Hold each lemon over the saucepan and cut between the membranes into sections allowing sections and any juice to fall into the saucepan. Squeeze juice from the membranes into the saucepan. Discard membranes. Add enough water to just cover. Bring to a boil, reduce the heat, and simmer until the garlic is very tender, about 20 minutes. Drain, then puree the lemons and garlic in a blender and measure out 1/2 cup for the vinaigrette. Reserve remaining lemon-garlic puree for another use.
In a medium bowl, whisk together the 1/2 cup of lemon-garlic puree with the vinegar, herbs, salt, and pepper, until the salt is dissolved. Slowly whisk in olive oil in a thin stream, and continue to whisk until fully emulsified; set aside.
Prepare the chickens: In a small bowl, combine the salt, rosemary, thyme, garlic, marjoram, sage, peppercorns, and shallots; mix well. Place chickens in a shallow baking dish. Sprinkle herb mixture over, and rub until well coated. Cover and refrigerate for at least 6 hours and up to 12 hours.
Cook the chickens: Preheat the oven to 500 degrees. Heat 3 tablespoons olive oil in each of two large ovenproof saute pans over medium-high heat. Add 2 chicken halves to each saute pan, skin-side down, and cook until brown, 3 to 4 minutes. Turn the chickens, and cook 3 minutes more. Turn the chickens again, so that they are skin-side down, and weigh each of them down with an ovenproof saute pan. Transfer to oven, and roast until the skin is crisp and the juices run clear, 8 to 12 minutes.
Prepare the vegetables: Slice the spring onions thinly on the diagonal, and rinse well. Snap off ends of the asparagus. Slice on the diagonal into 1/2-inch-thick slices up to the tips. Set the tips aside.
While the chickens are roasting, cook vegetables. Heat olive oil in a saute pan over medium heat. Add spring onions and the asparagus stalks, and cook until soft, 3 to 4 minutes. Season with salt and pepper. Add 2 tablespoons water, cover, and cook until crisp-tender, 3 to 4 minutes. Add asparagus tips, tarragon, and butter. Cook until the tips are heated through.
To serve, place the potatoes and vegetables on either side of the plate, the chicken in the center, and drizzle all over with the vinaigrette.