In a small saucepan, melt the butter over medium heat. Add the maple syrup, grainy and Dijon mustards along with the lemon juice, kosher salt, and pepper. Whisk until the glaze is fully emulsified and heated through.
Preheat the oven to 400°F. Peel the carrots and trim the green tops. Arrange the carrots on a baking sheet. Drizzle most of the glaze over the carrots, reserving 1 or 2 tablespoons for later. Toss the carrots in the glaze and then spread out on the baking sheet.
Roast the carrots for 30 to 35 minutes until tender. Check the carrots periodically to give them a stir and check that the glaze isn't burning. If the glaze begins to darken but the carrots aren't done, cover the baking sheet in foil and continue roasting.
The carrots are done when fork tender. Transfer the carrots to a serving platter and drizzle with the remaining glaze. Enjoy!