Add macaroni to 6 cups of salted boiling water. Cook until just barely underdone.
While macaroni is cooking, add condensed soup, very finely chopped onion (optional) and cheddar cheese in microwave safe 2-3 qt. casserole dish. Place in microwave oven on high for 3 mins. Check mixture and stir. If more time is needed, set oven at 1 min. intervals. Check after each addition until cheese mixture is melted.
Add cooked macaroni to cheese mixture and fold ingredients together.
Melt butter in small saucepan. Add bread crumbs to butter and stir together. Spread crumbs on top of macaroni & cheese mixture. Sprinkle parm. cheese over bread crumbs.
Place in preheated 350 deg. oven for about 30 to 45 mins. until heated through. Let set for approx. 10 mins. and serve.