The trick to this easy onion pie recipe just like in lots of onion recipes is to make sure you caramelize the
onions - this requires slow cooking. I usually put the burner on just above low and clean up the kitchen as the onions are cooking so I can watch them.
Cook the onions slowly in the butter and oil for 30 minutes, stirring occasionally. They should caramelize. Add flour and stir, cooking for two more minutes. Add milk and cream and cook another two minutes. Remove from heat.
Beat the eggs in a bowl, adding nutmeg and salt and pepper. Stir in the onion mixture and pour the resulting mixture into the pie shell. Bake at 400° F for 25 minutes.