Marinate the beef with the onion, salt, pepper, and garlic powder, at least 4 hours or overnight.
Place in a slow cooker and cook on low for 8 hours. It will be fall-apart fork tender when you are done.
Once the beef is cooked, use some tongs and place the beef in a bowl. Discard the juices.
Pull apart with a couple of forks, taste, and season with any additional salt and pepper should you desire.
When you are ready to make the sandwiches, butter each slice of bread.
Add 1 slice of the buttered bread, butter side down, on a preheated skillet on medium heat.
Add 2 slices of the cheddar cheese to the top of the bread, covering the bread, then add as much shredded beef as you want.
Top that with the remaining 2 slices of cheese, then top with the remaining slice of bread, butter side up.
Cook the sandwich for a few minutes, gently lifting to make sure you are getting that golden crust you want in a great grilled cheese sandwich.
Once golden, carefully flip over, and cook on the other side.
Lower the heat a bit, and if you have a sandwich press, at it to the top of the sandwich.
If you do not have a sandwich press, add a heavy pot to the top, or add a bit of pressure with a spatula.
Cook another few minutes, and once that side is golden, remove it and get ready to plate.
Slice the sandwich in half, on a diagonal.
Poke a few pickles with a toothpick and stack on top of the sandwich.
Marinate the chuck roast for at least four hours or overnight before cooking so the salt and aromatics fully penetrate the dense muscle fibers. Chuck roast is an economical cut that benefits immensely from long cooking times. When slow cooking one pound of beef on low heat for eight hours, the collagen breaks down completely into gelatin, yielding maximum moisture without requiring expensive cuts like ribeye or tenderloin. To adapt this dish for dairy-free diets, replace the cheddar with aged sheep-milk cheese or plant-based melting alternatives that mimic dairy fat content at a one-to-one ratio. For those avoiding gluten, swap the rustic bread for certified gluten-free sandwich loaves, keeping in mind that alternative grains often require an extra minute in the skillet to achieve structural integrity. Fresh onions yield a sharper flavor and higher water content during the multi-hour braise compared to dehydrated onion flakes, which lack fresh enzymatic activity and deliver a concentrated, flat sweetness. Cook the assembled sandwich on a preheated skillet over medium heat. Pressing down with a heavy pot or a sandwich press ensures even contact with the hot metal surface. Watch closely for a deep golden crust on the exterior bread before flipping. That visual cue prevents burning while guaranteeing the internal cheddar layers melt fully into the shredded meat.