Heat the oven to 425°F. Place the 6 medium carrots, peeled and cut on the diagonal into 1-inch pieces, and 2 large beets, trimmed, peeled, and cut into eighths, on a parchment-lined baking sheet.
In a bowl, combine 1/4 cup maple syrup, 2 tablespoons olive oil, 2 tablespoons yellow miso, 1 tablespoon chopped fresh thyme, 1 tablespoon chopped fresh rosemary, 2 teaspoons kosher salt, and 1 teaspoon black pepper. Whisk until smooth.
Pour the maple-miso mixture over the carrots and beets and toss until well coated. Spread the vegetables into an even layer.
Roast the beets and carrots until fork-tender, about 30 minutes.
Remove from the oven and toss with 3 tablespoons lemon juice. Transfer to a serving dish, sprinkle with additional fresh thyme and rosemary, and serve.