Preheat the oven to 350 degrees F (175 degrees C).

Place the carrots and onion into a large saucepan and fill with about 1 inch of water. Bring to a boil, then steam over medium heat until carrots are tender enough to pierce with a fork, 5 to 10 minutes. Drain and allow to cool slightly.

In a large bowl, stir together the mayonnaise, sugar and cheese. Stir in the carrots and onion until evenly coated. Transfer to a casserole dish. Top with crumbled crackers.

Bake for 20 to 30 minutes in the preheated oven, until sauce is bubbly and the top is toasted.