Cauliflower-and-Cheddar Soup

Since I called out one of my sisters for not digging soup, here's one for my other sister who is in love with Zuppas cauliflower soup. This recipe comes from Everyday food and I've noted some changes I made. Get past the name and you've got a yummy, creamy soup.
*3 TBSP. unsalted butter
*1 medium yellow onion, diced
*1 medium head cauliflower, trimmed and cut into 1 1/2 inch pieces
*1/8 tsp. red pepper flakes
*4 c. low-sodium chicken or veggie broth
*1 c. sharp white cheddar, grated
*1/2 c. pepper-jack cheese, grated
*coarse salt and ground pepper
*1/8 tsp. cayenne pepper

In a large pot, melt butter over medium.  Add onion and cook, stirring occasionally, until softened (5 minutes). Add cauliflower and red pepper flakes and cook until just beginning to brown (10-12 minutes). Add broth and bring to a boil over high.  Reduce heat and simmer until cauliflower is very tender, 20 minutes.  Use immersion blender or a blender (working in batches) to puree mixture.  Return to heat and add cheese.  Stir until melted and well blended.  Season with salt and pepper, serve sprinkled with cayenne.
This is not a spicy soup, but the peppers help to add flavor.  If you have some roasted garlic on hand, I think a clove or two added with the broth would be delicious.  We served it with hot bread sticks, and a plate of carrots and broccoli because it was as tasty as a cheese dip.