Peel and cut: Peel the carrots and cut them into 2-inch pieces, halving thick ones lengthwise so all pieces are about the same thickness — about 10 minutes of prep. Heat the oven to 425°F and line a rimmed baking sheet with parchment paper.
Toss: Toss the carrots with the olive oil, kosher salt, and black pepper in a large bowl until evenly coated.
Spread: Spread the carrots in a single layer on the prepared sheet, leaving space between pieces so they roast instead of steaming.
Roast: Roast for 20 to 25 minutes, turning once halfway through, until the thickest pieces are fork-tender and the edges are browned.
Glaze: Whisk the maple syrup and melted butter together, drizzle over the hot carrots, and toss to coat.
Finish in the oven: Return the pan to the oven for 3 to 5 minutes, until the glaze clings and darkens slightly at the edges — watch closely, because maple syrup goes from caramelized to scorched fast.
Serve: Scatter the thyme leaves and a pinch of flaky salt over the top and serve warm.