I think pastry crust would be a bit heavy for these, so I tend to use plain bread. These are easiest to eat if they're baked in bite sized mini cupcake tins, with a toasted bread crust holding all the onions in.
To make these, you will need:
16 slices of bread
3 large onions (I use yellow, whether regular or sweet)
2-3 T butter
a splash of apple cider (maybe 1/4 cup, more if you want more apple flavor to brighten the onion)
1/2 C or more of beef stock, preferably low sodium
4-8 oz shredded white cheese, depending on how much you like to add - I suggest mozzarella, provolone, swiss, havarti, or a combination of any of those
a few grinds of black pepper
bay leaf (optional)
Step 1: Make crust
Depending on your preference, you can either roll the circles flat with a rolling pin or squish them flat as you press them into the muffin tins. Either way, lightly butter the bottom side to keep them from sticking, and to add a nice golden brown color to the crust.