Place a 1-gallon resealable plastic bag in a large bowl, open up the bag and roll down the sides. Add the chicken thighs to the bag. Then add the garlic, shallots, vermouth, 1/2 cup olive oil, 1/4 cup of the Dijon, lemon juice, thyme, crushed red pepper and 2 teaspoons salt. Close the bag tightly, squeezing out any excess air. Use your hands (on the outside of the bag) to mix all the ingredients together, making sure to completely coat the chicken in the marinade. Place the bag back in the bowl and refrigerate for 2 hours.
Preheat the oven to 425 degrees F. Remove the marinated chicken from the refrigerator and allow it to come to room temperature while you roast the vegetables.
Slice the leeks into 1/2-inch rounds and place in a bowl of water to remove any dirt. Drain and dry with paper towels.
To a 9-by-12-by-3-inch baking dish, add the cleaned leeks, potatoes, red onions, 1 tablespoon olive oil and a generous sprinkle of salt. Toss together so everything is evenly coated. Roast for 35 minutes, or until the vegetables are tender and browned in spots but the potatoes have not split. Remove from the oven and set aside until the chicken is ready to add to the baking dish.
While the vegetables are roasting, use to tongs to remove the chicken from the marinade to a baking sheet, making sure to wipe any sliced shallots or garlic back into the marinade. Next, set a large strainer over a large bowl and strain the marinade; reserve the liquid and solids.
Sprinkle the chicken all over with salt. Heat 1 tablespoon olive oil and 1 tablespoon of the butter in a 12-inch skillet over medium-high heat. Sear the chicken on both sides, 3 minutes per side; work in batches if necessary, adding more oil and butter as needed. Remove the chicken to a clean baking sheet and reserve. Degrease the saute pan, wiping out any dark bits, and then melt 1 tablespoon butter over medium heat. Add all the solids from the marinade; saute until the shallots have just begun to soften, 2 to 3 minutes. Stir in the flour until completely incorporated, and then stir in the reserved marinade liquid and the chicken stock. Bring to a boil and cook for 2 minutes; for food safety reasons, it is important that the mixture comes to a full boil for a couple minutes.
Pour the sauteed shallot mixture evenly over the roasted vegetables in the baking dish. Place the chicken pieces on top and return to the oven for 10 to 15 minutes, until the chicken is almost cooked through (about 150 degrees F).
Meanwhile, in a small bowl, mix together the breadcrumbs, tarragon, crushed sea salt and melted butter.
Remove the chicken from the oven, slather with the remaining 1/4 cup Dijon mustard, and top with the breadcrumb mixture. Return to the oven for an additional 12 minutes, until the chicken has finished cooking and the breadcrumbs have browned on top. Serve immediately.
If Peewee Dutch Yellow potatoes are unavailable, substitute baby Yukon Gold potatoes, cut into small wedges. Fresh breadcrumbs are best here, but you can substitute panko breadcrumbs as well. To make fresh breadcrumbs for this recipe, place one-quarter of a small sourdough boule in a food processor and process until to coarse crumbs. Fresh breadcrumbs can be stored in an airtight zip-top bag in the freezer. Just remove the amount you need and defrost before using.