From the [1] comments, an onion tart recipe via our friend Jeff:

Onion Tart
- Tasteful Treasures, Friends of Fremont Library Cookbook
9” pie crust, unbaked
3 large onions, thinly sliced
3 large eggs, beaten
1/8 tsp pepper
1 1/2 cup light cream
3 Tbsp butter
3/4 tsp salt
Fry the thinly sliced onions gently in butter. Do not brown.
Arrange the onions in the pastry case. Mix the eggs, salt
and pepper with 1/4 cup of the cream. Heat the remaining
1 1/4 cups cream, add to the egg mixture and pour
over the onions. Bake at 350 degrees for 30 to 40 minutes,
or until a knife comes out clean.

thanks! And thanks Lenore for the prompt!