Preheat oven to 350 degrees F (180 degrees C).
To make the pie crust, mix Milk (1/4 cup) and Distilled White Vinegar (2 tablespoon). Place in the freezer until it is lightly frothy.
Mix together All-Purpose Flour (2 cup) and Salt (1/2 teaspoon), then add in Vegetable Shortening (1 cup).
Pour the milk mixture into the flour mixture and stir until all holds together. You may need to add a couple of extra tablespoons of cold water to make all cling together nicely, just add a spoonful at a time until it holds together as you are stirring. Don't knead; you want to mix and handle as little as possible.
Roll out crust and cut out circles using a jar or glass as a cutter.
Place circles into muffin tins, pressing them in to fit.
Prick the bottom of the crusts with a fork.
Bake pies shells for 5 minutes, then take out of oven.
Mix cooked Beef Chuck (1) with enough Gravy (1 1/2 cup) to make a really moist and soupy mixture. Reserve 1 - 1 1/2 cups for topping each muffin.
Put 1-2 tablespoons of shredded beef and gravy mixture in the bottom of each crust.
Sprinkle with a teaspoon (or desired amount) each of White Onion (1), Frozen Green Peas (1/4 cup), and Frozen Corn (1/4 cup). Divide the remaining Gravy (1 1/2 cup) evenly over the top, around 1 tablespoon each.
Evenly divide the Mashed Potatoes (2 cup) between the cups. If you want to make your potato topper look nice, put the potatoes in a cake decorating bag and use a large tip to pipe onto the muffins. If you do this, make sure to mash your potatoes with an electric mixer, and mix them until they are completely soft; no lumps.
Bake at 350 degrees F (180 degrees C) for 30-40 minutes; or until muffins are bubbly and potatoes are lightly brown. Serve warm.