Place onions and butter in a large microwave-safe bowl; cover and microwave on high for 2-3 minutes or until onions are tender. Stir in the mustard, poppy seeds and lemon juice. Cover and microwave on high for 1 minute or until heated through.
Spread onion mixture over cut sides of croissants. On bottom halves, layer ham and cheese; replace tops.
Wrap each sandwich in foil. Place on a baking sheet. Bake at 350° for 20-25 minutes or until cheese is melted.