Saute chopped onion and garlic in a small pan for 2-3 minutes until translucent and golden on medium. Add in a can of tomato/ green chile and let simmer on medium-low.
In a large nonstick skillet melt 1T butter on medium-low. Whisk 5 teaspoon flour with ½ cup milk and add to the pan. Slowly add in the remainder of milk and half and half. Whisk constantly for 7-10 minutes until sauce thickens.
Turn off heat and add in garlic, pepper, and pepper jack cheese. Whisk to melt the cheese and add in ¼ cup of cheddar at a time whisking constantly to melt. Continue until all the cheese is added. Add in tomato/onion mix and spinach. Serve immediately or transfer to a fondue pot or crockpot until ready to serve.