Melt butter in a large saucepan and sauté chopped onions until soft (alternatively, you can also do this in the slow cooker).

Combine sautéed onions, spinach, artichokes, cream cheese, sour cream, 3/4 cup Jack cheese, 3/4 cup Parmesan cheese and hot sauce in the slow cooker.  Unlike traditional slow cooker recipes, you'll want to stir the dip occasionally and keep an eye on it so it doesn't brown on the edges.

If you prefer, you may pour the mixture into a 1/2 quart casserole dish and top with remaining cheeses. Bake for 10 minutes at 350F or until cheese melts and turns golden brown.