Prep the spinach. If using frozen spinach, thaw fully and squeeze out as much water as possible with a clean towel. Dry spinach is key for a thick, creamy dip.
Chop the artichokes. Drain well and chop into bite-size pieces.
You want texture, not mush.
Load the slow cooker. Add cream cheese, sour cream, mayonnaise, mozzarella, Parmesan, garlic, onion powder, red pepper flakes, salt, and black pepper. Top with spinach and artichokes.
Cook low and slow. Cover and cook on Low for 2–3 hours or on High for 1–1.5 hours, until the cheese is melted and everything is steamy.
Stir and brighten. Stir well to combine. Add lemon juice and taste for salt, pepper, and heat.
Adjust as needed.
Keep warm for serving. Switch to Warm. Stir occasionally to prevent hot spots. If it thickens too much, splash in a tablespoon of milk or cream.
Serve. Set out with chips, bread, or vegetables.
Garnish with a sprinkle of Parmesan or red pepper flakes if you like.