MAKE 20 evenly spaced small cuts, about 1 ½ inches deep, all over pot roast, using the tip of a sharp paring knife. Insert garlic cloves as deep into the cuts as possible.
SEASON the roast on all sides with salt and pepper.
HEAT a 6-quart Dutch oven over high heat. Add oil, and when it is hot, sear the roast until it is very well browned on all sides, 4 to 6 minutes per side (don’t be afraid to let the roast get very brown – this is where a lot of the flavor comes from).
TRANSFER roast to a slow cooker, then carefully add stock and close lid. Set slow cooker to high and cook for 4 hours (and up to 6 hours depending on your machine). When roast is done, it should be fork tender.
PULL meat apart into thin shreds, using two forks, and mix with accumulated drippings in bottom of pot. Allow meat to cool slightly and absorb juices before assembling poboys. (Roast can be cooked and shredded up to 1 week in advance and refrigerated until ready to serve; if prepared in advance, it will need to be rewarmed, covered and in a low oven, before assembling poboys.)
HALVE each bread piece lengthwise and spread top and bottom halves liberally with mayonnaise. Place bottom halves on a baking sheet and spoon shredded meat over them, drizzling it with extra drippings. Top meat with cheese slices.
BROIL bottom halves of bread with meat and cheese in oven just until cheese is melted, 2 to 3 minutes.
REMOVE from oven.
TOP with lettuce, sprinkle with hot sauce (if desired), add top halves of bread to each sandwich and serve immediately.