Wash wild rice in three changes of hot tap water; drain. Add rice, salt, and water to saucepan and heat to boiling. Reduce heat and cover. Simmer until rice is tender and has absorbed the water, about 30 to 45 minutes; for al dente rice, drain after 30 minutes. Rice should be tender but not mushy. Some kernels will be open, and others will be just tender. Set aside.
Melt the butter in large heavy pot. Add the onion and mushrooms, sautéing for 5 minutes or until vegetables are cooked.
Add the broccoli, carrots and chicken broth. Simmer until vegetables are tender.
Place the vegetable mixture into a blender or food processor and process until smooth. You may need to do this in batches. Add flour and cream; blend until smooth. Return pureed vegetables to pot.