How to Make It
In a large glass dish, combine 1/4 cup of the olive oil with 1/4 cup of the vermouth, the sliced onion, thyme sprigs and chiles. Add the chickens and turn to coat. Cover and refrigerate for 5 hours.
Trim and peel the 2 yellow and 2 red onions, keeping the root ends intact. Cut each onion into 8 wedges. Trim the leeks, keeping the root ends intact, and cut each one lengthwise into sixths.
Steam the potatoes over boiling water until still slightly firm, about 10 minutes. Let cool slightly, then peel and cut each one lengthwise into 6 wedges.
Preheat the oven to 450°. Heat 2 tablespoons of olive oil in each of 2 large skillets. Add the potatoes to 1 skillet and the onions and leeks to the other; cook over moderately high heat, turning, until golden, 6 to 8 minutes. Transfer the vegetables to a roasting pan. Sprinkle with salt, pepper and 2 1/2 teaspoons of thyme.
Return the skillets to moderately high heat. Scrape the marinade off the chickens. Add them to the skillets, skin side down, and cook over moderately high heat until the skin is deep golden and crisp, about 5 minutes. Set the chickens on the vegetables, skin side up, and season with salt and pepper.
Melt 1 tablespoon of the butter in a small skillet. Add the shallots and cook over moderately high heat until translucent. Add the remaining 1/4 cup of vermouth and cook until reduced by half. Remove from the heat and stir in the mustard, tarragon and the remaining 1/2 teaspoon of thyme. Slather the chickens with this mixture.
In a small skillet, melt the remaining 2 tablespoons of butter. Add the bread crumbs and parsley, season with salt and pepper and toss to combine. Spoon the crumbs over the chicken and add the chicken stock to the baking dish. Roast in the oven for about 25 minutes, or until the crumbs are golden and the chicken is cooked through. Serve at once.