Spray the stoneware of your slow cooker with nonstick cooking spray.
Melt the butter in a medium skillet over medium-high heat.
Add the onion and cook until tender, stirring frequently, for 3 to 5 minutes.
Add the garlic and cook for an additional 30 seconds until fragrant.
Transfer the onion mixture to a large bowl.
Add the drained spinach, chopped artichokes, cream cheese, mayonnaise, Parmesan cheese, and cheddar cheese to the bowl.
Stir until everything is well combined.
Spoon the mixture into the prepared slow cooker.
Cover and cook on High for 1 hour, stirring after 30 minutes to ensure even melting.
Sprinkle with toasted pecans just before serving.