This recipe comes from your dad's biological half-sister Barbara Geiger. We didn't meet her until she was almost 50 (she was almost 20 years older than your dad), but she was a great person. She died a number of years ago of pancreatic cancer. I got this recipe from her when we spent Thanksgiving with her and her family.
2 lbs small white onions or an equal portion of frozen onions
1/4 cup flour
1/4 cup butter
1 cup cream
1 cup onion water or 1 cup milk if using frozen onions
1/4 lb. grated sharp cheddar
1/2 tsp salt or to taste
1/8 tsp white pepper
generous pinch of cayenne
Bread crumbs & 1 tbl butter, cut in pieces (optional)
For fresh onions: Cut an X in the bottom of each onion. Cook in boiling water until barely tender. Drain, saving 1 cup of the water. Place onions in a greased, 2 qt oven-proof casserole. Make sauce by cooking butter in a sauce pan until melted, add the flour and cook at least 3 minutes. Add liquids slowly while stirring constantly. Bring back to the boil, reduce heat and add cheese, salt & peppers. Pour over onions. Bake for 20-25 minutes. Top with bread crumbs and dot with butter. Broil,watching carefully, until browned.