In a large pot, add the butter and oil. Heat to medium. Once butter melts, add the onion, jalapeño, garlic and poatotoes. Season lightly with salt and pepper. Saute for 6-8 minutes.
Add the milk and chicken broth. Stir well to combine. Cook and a slight simmer just until potatoes are fork tender.
Stir in the mashed potatoes and diced green chiles. Continue cooking for another 7-9 minutes. Taste for salt and pepper.
To serve, add 1/3 cup of Cabot shredded cheddar cheese to the bottom of the bowl. Ladle in the hot soup on top. Garnish with croutons,more cheese, jalapeño slices and Spanish chorizo. Yields 6-8 servings.