Pat the beef dry with paper towels and season generously with salt and pepper.Add the beef to the skillet sear until golden-brown, about 10 minutes per side; transfer to the bowl of the slow cooker. Reduce heat to medium. Add the onions and cook until translucent, about 5 minutes.Add the stock, garlic to the slow cooker and cover with the lid. Cook on low for 7 hours (or high for 4 hours).When done, remove the meat from the cooker and shred with two forks. Chop to desired texture and set aside.Meanwhile, melt the butter in a large Dutch oven of heavy skillet on medium heat until foaming. Add the flour and cook, stirring constantly, until it turns a shade of amber and gives off a nutty smell.Pour the cooking liquid from the slow cooker into the skillet with the roux. Bring to a boil then reduce heat to a simmer. Cook the gravy until thickened, 10 to 15 minutes. Season with salt and pepper to taste. Stir the chopped meat into the gravy, taste, and adjust any seasonings if necessary.To assemble the po’ boys, slather the bread with a generous amount of French sauce. Top with shredded meat and gravy. Serve with shredded lettuce and tomatoes, if desired.