Step 1

For the roast beef: Bring 1 gallon of water to a boil. Place a dishtowel on the bottom of another large stockpot to prevent the jars from moving. Add 4 inches of the hot water to the second pot and bring back to a boil. Lower the jars upside-down along with the lids into the water and boil for 15 to 20 minutes. Remove the jars from the water and cool completely.

Step 2

Fill the jars with the chuck roast, leaving 1 inch of space at the top. Next add a piece of onion to each jar, followed by 1 teaspoon of salt. Then fill each jar with some of the boiling water. Remove any air bubbles by inserting a butter knife into the jar and pressing out the bubbles. Wipe the rims of the jars with a damp cloth and tighten the lids onto the jars.

Step 3

Bring another gallon of water to a boil. Place another clean dishtowel in the bottom of an empty stockpot. Lower the jars using a jar lifter. Add enough of the boiling water to cover the jars by at least 1 inch. Increase the heat to high and bring the water back to a boil; cover and cook until the temperature reaches 180 degrees F, and cook, maintaining this temperature, for 30 to 40 minutes. The jars are sealed when the center of the lid is pushed in.

Step 4

Place the jars on a cooling rack and rest, undisturbed, for at least 12 hours and up to 24 hours.

Step 5

Label each jar with the date and store in a cool, dark place.

Step 6

For the sandwich: In a large saucepan, bring the beef broth to a boil.

Step 7

In a small bowl, mix the cornstarch with 1/2 cup cold water. Whisk the cornstarch mixture into the broth, and then cook until the mixture thickens, about 2 minutes. Remove from the heat and keep warm for later use.

Step 8

Add the potatoes to a large pot of salted water. Bring to a boil and cook until tender, about 15 minutes. Drain and return the potatoes to the pot. Add the milk and margarine, and mash together until smooth. Season the mashed potatoes with salt and pepper.

Step 9

In a medium saucepan, heat 4 containers of the beef until warmed through.

Step 10

To serve, place a slice of bread on a plate, then a scoop of the hot beef on top, followed by a scoop of the mashed potatoes. Repeat for the remaining slices of bread, the hot beef and mashed potatoes. Ladle the gravy generously over the top, and serve.