Portobello Mushroom Soup

1 pound salted butter
1 pound leeks
1 pound yellow onions, diced
2 pounds portobello mushrooms, diced
3 ounces port wine
1 quart Half &Half
3 quarts Chicken stock
1/2 flour

Melt butter in a heavy stock pot. Add leeks and onions, saute. Then add mushrooms, for a shorter time. Add flour (it will act like roux). Heat chicken stock, Half &Half and port. Pour into the vegetables, stir. Let simmer and reduction will occur.