Heat a deep skillet on medium heat; add the oil and butter. When the butter is melted but not burned, add the onions. Lower the heat if the butter starts to burn, and stir for a couple of minutes. Add the carrots and continue to stir for 6–7 minutes, depending upon the width of the carrots. Add the tarragon leaves, ripping them apart as you add them to the skillet. Drizzle the maple syrup into the skillet, add salt to taste, and stir the carrots until they are uniformly glazed, cooking approximately 3–4 minutes more. Serve as a side dish.