Put the onions in a medium pot and add about 1/2 cup cold water and a sprinkle of salt. Put the pot on medium heat and simmer until almost all the water has boiled off. Add several tbsp butter and continue cooking. Eventually all the liquid will evaporate and the onions will start frying gently. Keep going, stirring frequently, until the onions are well-browned. Add the cognac and cook until all the liquid has boiled off again. Start to finish, this step will probably take 45 min to 1 hr.

Remove the onions from the pot and set them aside. If there are any burned bits on the bottom of the pot, scrape them off and wipe with some paper towel. Season the meat well with salt and pepper. Add a splash of veg oil to the pot and sear the meat until browned on all sides. This should take about 8 to 10 minutes. Remove the meat.

Return the onions to the pot and lay in the thyme sprigs. Top with the meat, then add 1 cup of the beef stock (or enough to cover the meat by 2 or 3 cm). Cover, then cook in an oven preheated to 325 F until fork tender. This should take about 2 to 2 1/2 hrs.

Carefully pull the pot from the oven and remove the meat. Discard the thyme and skim as much fat as you can from the surface of the broth. Add the remaining beef stock, carrots, celery, and Worcestershire sauce, as well as a pinch of salt and several grinds of black pepper. Put the pot on the stove on medium heat and simmer until the veg are tender, about 30 minutes. Whisk the cornstarch with a few tbsp cold water and stir it into the pot. Simmer a final 2 to 3 min to thicken.

Transfer the gravy to a deep baking dish. Carve the meat into bite-sized chunks and nestle it into the gravy. Layer the bread (garlic side up) on top. Put the dish into the oven and broil for 2 or 3 minutes to toast the bread slightly. Grate the gruyere and sprinkle it on top of the bread, and broil about 5 minutes more until melted and bubbly. Serve piping hot.