In a cast iron pan or medium skillet (preferably one that is both stovetop and oven-safe), saute the spinach and garlic in 2 tablespoons of butter until the spinach is wilted and the garlic fragrant.
Mix together cream cheese, 1 cup of jack cheese, ½ cup parmesan cheese, sour cream, and artichoke hearts. Combine with the spinach mixture.
Spread ingredients in a single layer in the skillet or a baking dish.
Bake for 20 minutes. Remove from oven to cover with remaining 1 cup of jack cheese and ½ cup of parmesan.
Bake until brown and bubbly or broil for just a couple minutes longer to melt and brown the cheese.
Serve hot with tortilla chips, baguette slices, or thick crackers.
Notes
When mayonnaise is heated, the oils separate. This means that mayo might be a great creamy component in cold dips but this warm, melted cheese version is best as a spinach artichoke dip without mayonnaise. The cream cheese and sour cream provide all the richness and creaminess and their structure doesn't change with heat.
Wilting the spinach takes just a minute or two and the garlic will sauté just as quickly. Don't burn the garlic; its bitter flavor will ruin the dip!
If you don't have an oven-safe skillet, transfer the dip to a 9x9 or 9x13 baking dish. This is also a great option if you wish to double the recipe.