This soup can be made ahead and thinned with milk when reheating.
Melt butter in a large Dutch oven over medium-high heat; add mushrooms and onion, and sauté 5 minutes or until tender.
Add chicken broth and next 9 ingredients; simmer 10 minutes or until orzo is tender.
Stir together milk and flour in a small bowl; gradually stir into chowder, and simmer 5 minutes.
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