PREP BEEF: Remove beef from packaging and pat dry with a paper towel. Season the roast all over with salt and pepper to taste. I add 2 teaspoons fine sea salt (or 1 tablespoon Kosher salt) and 1 teaspoon pepper. Gently press seasonings into the meat.
CHAR BEEF: Warm 2 tablespoons of olive oil in a large Dutch oven (or oven-safe heavy-bottomed pot) over medium-high heat. Once you can feel the heat when you hold your hand 6 inches above the pot, load in the beef. Brown for about 5 minutes or until you get a nice crust and it releases easily from the pan when lifted. Flip and brown for another 5 minutes. Don't skip this step. We are adding layers of flavor here. Once the beef is nicely seared with a good crust, remove it from the Dutch oven and place it in the slow cooker.
ONIONS: Reduce heat to medium and add the remaining 2 tablespoons of olive oil. Add the thinly sliced onions and cook, stirring occasionally, until the onions are translucent, about 3-5 minutes. Sprinkle in the seasonings (1 tablespoon beef bouillon powder, 2 teaspoons garlic powder, 2 teaspoons onion powder, 1/4 teaspoon thyme) and stir until fragrant, about 30 seconds. Add the Worcestershire sauce. Stir to combine. Pour in the beef broth and deglaze the pan by scraping up the browned bits at the bottom of the pan with a wooden spoon or spatula as it bubbles. (This is where we're going to get loads of flavor.)
SLOW COOK: Pour all the contents of the pot into the slow cooker, scraping the bottom of the pan. Add bay leaf. Cover and cook for 7-9 hours on low or until meat is fall-apart tender. I don't recommend cooking on high, as it doesn't get nearly as tender. (But if you do, about 5-7 hours is how long it will take on high.) Pull out some butter now too so it will be nice and soft come time to make the sandwiches!
MAKE SANDWICHES: Use a sharp bread knife to slice sub rolls in half. Butter the insides of both pieces and lightly sprinkle with garlic powder (optional, but yum!). Place on a sheet pan and broil for 20 seconds, watching to avoid burning. Remove. Use a slotted spoon to remove meat from the slow cooker (shake off excess liquid) and place it on the bottom half of the rolls. Add cheese on top. Return to the oven and broil for 30 seconds to a minute or two, until cheese is melted and tops are light golden brown. Watch very closely-- they can go from perfect to burnt in seconds! Sandwich the two sides together. Spoon out liquid from the slow cooker into bowls and serve alongside (for dipping) with the sandwiches. Enjoy promptly out of the oven.