Cook sweet potatoes in boiling water to cover 20 minutes or until barely tender. Drain and let cool slightly. Peel potatoes, and cut crosswise into 1/2" slices. Layer potatoes in a lightly greased 13" x 9" baking dish or oval gratin dish.
Melt butter in a saucepan over medium heat; add onion and garlic, and sauté until tender. Sprinkle flour over onion mixture; cook 1 minute, stirring constantly. Gradually add half-and-half, stirring constantly until thickened and bubbly. Stir in salt, cumin, and pepper. Add cheese and chipotle peppers, stirring until cheese melts.
Pour cheese sauce over potatoes. Bake, uncovered, at 350° for 45 minutes or until lightly browned and bubbly. Let stand 15 minutes before serving.