My recipe for this soup is different in that it includes some potato. Most versions are a straight flour-bound (roux-based) soup prepared on the béchamel (all milk/cream) or velouté (stock with milk/cream) model. Although I do begin my soup with a roux, it is based more on the "potato-leek" model (substituting onions for leeks).
Many single vegetable soups (puréed or not) are based on this classic French soup of leeks/onions, potatoes, water or stock and sometimes cream. Darina Allen in her book Forgotten Skills of Cooking teaches that almost any vegetable can be made into a soup using this model. For these vegetable soups, the potatoes provide the thickening—adding not only body and a nice velvety texture, but a fuller flavor as well.
I have been making this particular soup for almost as long as I have been cooking so unfortunately I don't really remember much about its origins. But as I looked at other recipes and noticed the lack of potato, I began to wonder why I had included flour at all in my potato-leek-style version. As I was pondering this I remembered a conversation I had with my chef when I was a still a fairly young cook. The answer has to do with the fact that this isn't just a broccoli soup...it also includes cheese. Good quality natural cheeses often lack the stable melting qualities of artificial cheeses. Knowing this, I had asked my chef how to make a cheese based soup (with real cheese) that wouldn't curdle and break upon heating. Her answer was simple: start with a roux...just like the mornay sauce (béchamel with cheese added) that is used to make macaroni and cheese. Apparently I added a roux to my soup to accommodate the addition of the cheddar cheese.
I could of course have done away with the potato altogether, but as I mentioned before, I like the qualities it adds. Furthermore, by using a potato, I can use less flour. To me this is a good thing since flour-bound velouté-style soups are prone to being over-thickened and a bit gloppy. I devote an entire paragraph to my thoughts on what constitutes a properly thickened soup in my Cream of Wild Rice Soup post. I will only say here that if the soup mounds on a spoon, it is too thick. My Broccoli Cheese soup is only lightly thickened. If you like very thick soups, this one is probably not for you.
The only other thing I will add is that to make a truly fine Broccoli Cheese soup, you need to use a high quality, sharp cheddar. For years my favorite sharp white cheddar has been Black Diamond. But recently I have discovered a delicious cheddar from Milton Creamery in Iowa. It's called Prairie Breeze. It is an excellent cheese—award winning, in fact. And it is very good in this soup. If you can't get either of these cheeses, any nice sharp cheddar—or cheddar-like cheese—will do nicely. When made with good ingredients (and cooked with care), one taste will reveal exactly why this soup became so popular in the first place.
Broccoli Cheese Soup
1 large or 2 small heads broccoli (1 lb.), woody ends trimmed away and discarded
1 onion (about 8 oz.), cut in a 1/4-inch dice (1 1/2 cups)
4 T. unsalted butter
2 cloves garlic, minced
1/2 t. dry mustard
3 T. flour
6 c. chicken stock or water
1 lb. russet potatoes, peeled and cut in 1/4-inch dice (about 2 1/2 cups)
1 c. heavy cream (half & half or milk may be substituted)
6 to 8 oz. grated sharp white cheddar cheese, plus more for garnish
Cut the florets off of the broccoli stems and coarsely chop; set aside. If necessary, peel the stems. Cut the stems into a small, rough dice. You will have about 3 cups chopped florets and 1 1/2 cups diced stems.
Melt the butter in a large pot over medium heat. Add the diced broccoli stems and the onion along with a pinch of salt. Cover and sweat over low heat until tender—about 15 minutes. Add the garlic and mustard and cook briefly—just until the garlic is fragrant. Add the flour and cook a minute or two, stirring occasionally. Add the stock along with the potatoes and chopped broccoli florets and bring to a simmer. Simmer (uncovered) until the vegetables are tender—about 15 to 20 minutes. When the vegetables are tender, pass the soup through a food mill or purée one third to half of the soup in a blender. If you have an immersion blender, simply blend the soup until it is the texture you prefer. Add the cream to the soup and heat through. Remove from the heat and stir in the cheese. Taste and correct the seasoning with salt & pepper. Garnish each bowl with a sprinkling of cheese or pass extra cheese at the table.
Makes 2 to 2 1/2 quarts soup to serve 6.