Preheat the oven to 400 degrees

Dice the potatoes into 1/2 inch cubes.   Put the potatoes onto a parchment lined baking sheet.

Add the oil, garlic and salt.  Mix together to coat all the potatoes evenly.

Place in the oven on the middle rack for 45 minutes.  Make sure to keep an eye on them.

Remove from oven and let cool.

Peel the paper skin off of the tomatillos and cut in half, then spread them out on a parchment-lined baking sheet, cut side down.

Turn on the broiler and broil tomatillos for 6 – 7 minutes then flip over for another 3 – 4 minutes or until very soft and slightly burnt.

Add the onion to a pot of water and bring to a boil, then remove, let cool and add to a food processor.

When tomatillos are finished cooking, let cool and add to the food processor.

Cut the stems off the serrano then de-seed and add to the food processor along with the cilantro, garlic,  lime, and salt.  Puree until smooth.

Shred the cheese and set it aside.

Dice the onion and add to a 5 1/2 quart soup pot along with the oil and sauté on medium-low heat until onions are soft and translucent.

Add the water, potatoes, tomatillo salsa, lime juice, salt and pepper.

10. Let simmer on low heat.

Meanwhile, melt the butter in a large saucepan over medium-low heat.

When the butter is melted, add the flour and stir for about a minute.

Slowly whisk in the half and half a little at a time. It will clump and then smooth out.

When the half and half is hot, slowly add the cheese a little at a time.

When completely melted, add to the soup.

In a food processor puree in batches or use a handheld immersion blender.

Pour back into the soup and stir.

Serve it with tomatillo salsa, cilantro oil and sour cream if desired.