Heat the oven to 375°F and grease a 9-by-13-inch baking dish. Simmer the sweet potato rounds in a large pot of water for 7 to 8 minutes until the outer edges soften but the centers still offer resistance; drain carefully.
Reserve 1 tablespoon maple syrup. Whisk the remaining syrup with melted butter, water, lemon juice, salt and cinnamon.
Arrange the rounds in overlapping layers in the dish and pour the maple mixture evenly over them. Cover with foil and bake for 20 minutes.
Toss the pecan halves with the reserved syrup. Uncover the dish, spoon some hot glaze over the potatoes and scatter the pecans loosely across the top.
Bake for 12 to 15 minutes more until the potatoes are tender, the glaze is bubbling and the pecans are toasted. Rest for 10 minutes before serving.