In large sauce pan melt 2 sticks of butter and sauté the onion (Vidalia if possible) until tender, but not brown. Sprinkle in the flour and whisk until well blended. Slowly add the chicken broth and whisk all the time. Bring to a boil over medium heat. Add cut up cream cheese and whisk until smooth and melted. Add mushrooms and all seasonings. Taste and adjust if needed. Over lowest heat cook 5-10 minutes. Add the thawed peas and fold together.
Process the French bread into crumbs (food processor). Add 1/2 the crumbs to the pea mixture and fold together. Pour into 9x12 casserole dish. (It will be very full).
Bake at 350°F for 30 minutes until bubbly. Remove from oven, but do not turn off! Sprinkle with topping:
Melt the remaining stick of butter and combine with remaining bread crumbs. Add 1/2-3/4 Parmesan cheese (cheddar or Swiss would also be good).
Return to oven until topping is golden brown 10-15 more minutes.