Brussels Sprouts with Chestnuts and Honey Mustard Dressing

To make dressing: In a small saucepan, cover 1 lemon with water. Bring to a boil; reduce heat and simmer until soft, about an hour, turning lemon occasionally. Remove lemon and let cool. Cut lemon in half and with a teaspoon scrape out pulp, leaving just the peel; finely chop peel. Squeeze 4 tablespoons juice from remaining lemons. In a bowl, mix lemon peel, olive oil, Dijon mustard, honey, whole grain mustard, thyme, salt, and pepper. Whisk to combine. To prepare vegetables: With a paring knife, cut an X in the flat side of each chestnut shell. In a saucepan, cover chestnuts with water and bring to a boil; reduce heat and simmer until tender, about 15 minutes. Drain chestnuts. When cool enough to handle but still warm, peel chestnuts. With tip of paring knife, make a shallow X in the bottom of each brussels sprout. Bring a large pot of salted water to a boil. Add brussels sprouts and cook until tender, about 6 minutes. Drain and run under cold water until cool. In a large skillet, melt butter over medium heat. Add chestnuts and sprouts; season with salt and pepper to taste. Cook, turning occasionally with a spatula, until vegetables are browned, 15 to 20 minutes. With a slotted spoon, remove vegetables to a large serving platter. Whisk dressing and drizzle 1/4 cup over vegetables.