In a small bowl, whisk the yogurt with 1 tablespoon of warm water until smooth.
Add the yogurt mixture to the slow cooker along with the cream cheese, mozzarella, Parmesan, minced garlic, salt, pepper, and chopped artichoke hearts.
Stir until the ingredients are evenly combined.
Cover and cook on Low for 2 hours.
Stir the mixture until smooth and creamy.
Add the fresh spinach, chopped red onion, and fresh lemon juice.
Stir until the spinach begins to wilt.
Cover and continue cooking on Low for another hour.
Stir once more before serving.
Serve the Slow Cooker Spinach Artichoke Dip warm with chips, toasted bread, crackers, or fresh vegetables.