Preheat oven to 400°F (205°C). Line a large 18x13-inch rimmed half-sheet baking pan with parchment paper. In a glass measuring cup, vigorously whisk together pure maple syrup, Dijon mustard, whole-grain mustard, minced garlic, chopped fresh rosemary, chopped thyme, 1 tablespoon olive oil, smoked paprika, 1/2 teaspoon kosher salt, and black pepper until emulsified.

Place cubed sweet potatoes and broccoli florets in a large bowl. Drizzle with the remaining 2 tablespoons olive oil and 1/2 teaspoon kosher salt; toss well. Spread the vegetables evenly across the prepared sheet pan, leaving spaces for the chicken thighs.

Pat chicken thighs completely dry with paper towels so skin crisps properly. Nestle chicken thighs skin-side up between the vegetables. Use a silicone pastry brush to generously paint half of the maple Dijon glaze all over the top and sides of each chicken piece.

Transfer the sheet pan to the center oven rack and roast for 20 minutes. Carefully pull out the pan and generously brush the remaining glaze over the chicken and drizzle any remaining over the sweet potatoes. Scatter fresh thyme sprigs over top.

Return to the oven for an additional 12 to 15 minutes, until the chicken reaches an internal temperature of 175°F (79°C) and the sweet potatoes are fork-tender with blistered caramelized edges. (Optional: broil on HIGH for 2 minutes for deep mahogany blistering). Rest for 5 minutes, sprinkle with flaky sea salt, and serve hot.