Your recipe sounds wonderful. But I'd prepare it a bit differently. First, you can purchase truffle oil fairly cheeply if you look around. Saute you rshrooms in it to add a glorious, earthy flavor. Second. Before lightly browning the shrooms, cook, them with a lid on the pan to capture the well flavored steam that normally escapes into the air. Use that as part of the liquid stock to thin the roux. Then pour off the mushroom stock, add the butter, onions, and garlic to the pan and lightly brown. While they are browning, thin the roux with milk, the mushroom broth, and a bit of cream. After that, add the mushrooms, onions, and garlic. Add salt and pepper to taste. Your soup will come out very good.
This soup is even better when strong flavored mushrooms like portabella's are used.
Seeeeeya; Goodweed of the North