In a medium skillet, melt the butter over medium heat. Add the garlic and cook until fragrant, about 1 minute.

Increase the heat to medium-high and add the spinach. Cook, stirring occasionally, until the spinach wilts and cooks down, about 1 minute. Remove the spinach from the skillet and put it in a small strainer. Squeeze the excess juice from the spinach back into the skillet. Set the spinach aside.

Add the artichokes to the liquid in the skillet and cook over medium-high heat until the liquid is cooked off and the artichokes start to get a little color, 7 to 8 minutes. Remove from the heat. Let cool slightly, then coarsely chop.

In a 6-quart slow cooker, stir together the spinach, artichokes, cream cheese, sour cream, pepper jack, feta, parmesan, milk, salt, black pepper, and cayenne pepper. Cover and cook on low, stirring occasionally, until the dip is warmed through and thickened, about 2 hours.

Give it a good stir before serving in the slow cooker or transferring to a serving bowl. Serve with warm pita wedges, tortilla chips, or crackers.