Coarsely chop the spinach and drain the artichoke hearts. In a large bowl, beat together, mayonnaise, sour cream, and cream cheese until smooth. Stir in the onion, garlic and Dijon mustard. Gently stir in spinach, artichoke hearts and cheeses until well mixed.

Preheat the oven to 375 F. Cut the bagels in half and lay them out on a cookie sheet. Spread with about two tablespoons of the dip mixture. Bake for 7-8 minutes until cheese it melted and bubbly. If desired, place them under the broiler for the last minute or two until the cheese starts to brown, but watch them carefully. The are best when served immediately. The dip can be made a day or two ahead of time and spread on the bagels and baked right before they are served.