Saute 3/4 cup of the mushroom, seasonings and the onion in a medium saucepan on medium high heat until browned and soft (about 5-10 minutes). Add flour and butter to this mixture until the flour is lightly browned (about 5 minutes).
Meanwhile, saute the rest of the mushrooms in a separate pan until browned.
Add one addition of the half and half to the onion and mushroom mixture and stir until thick, add the rest. Pour mixture into a blender and blend until smooth.
Place the mixture back on the stove and add the rest of the mushrooms. Serve with sour cream.