Preheat oven to 350°F.

Heat oil in a large, nonstick pan over moderate heat. Sauté the shallot, carrots, celery, garlic and mushrooms, seasoned with salt and pepper, until soft. Add herbs, flour and butter and let cook, stirring occasionally, until the butter has melted and the flour has cooked a little bit (about 2 minutes). Add stock and let it cook for a few minutes to thicken, stirring occasionally.

Stir in meat and potatoes, season again with salt and pepper, and remove from heat.

Cut each sheet of puff pastry into 6 equal rectangles (this will give you 12 total). Press each piece of pastry into one cup of a muffin tin, making sure to flatten the bottom and leave the edges hanging out. Portion the pot pie filling into each of the prepared muffin cups. (I found that using an ice cream scoop gave me perfect portions for each cup).

Bake until the pastry has puffed and the edges are golden, around 20 minutes.