Preheat the oven to 375 degrees F. Butter a 9 by 13-inch baking dish.
In a large skillet melt the butter over low heat. Blend in the flour, cumin, garlic, powder, and salt. Add all of the half and half at one time. Turn the heat up to medium-high and cook a couple of minutes until the mixture thickens and bubbles.
Add 2 cups of the Mexican cheese and mix until the cheese is melted and blended in. Add the potatoes, peppers and onions mix in. Pour the potatoes into the prepared baking pan.
Place in the preheated oven and bake for 1 hour and 10 to 15 minutes or until potatoes are tender. Remove the potatoes and top with the remaining 2 cups of Mexican cheese and continue to bake for about 10 minutes or until cheese is melted.