In a skillet cook bacon until crisp. Drain on paper towels. Crumble bacon; cover and chill until ready to use. Cook onion in 1 tablespoon bacon drippings about 5 minutes or until tender.
In a 3 1/2- to 4-quart slow cooker combine onion, artichoke hearts, spinach, red sweet pepper, mayonnaise, cream cheese, blue cheese, garlic, and dry mustard. Stir to combine.
Cover and cook on low-heat setting for 3 to 4 hours or until cheese melts and mixture is heated through. Stir in bacon. Serve with assorted crackers.