Sprinkle the shanks with salt and leave them aside. Mash the garlic cloves in a bowl, add the olive oil, lemon juice, ketchup, Worcestershire sauce, mustard, ground black pepper and cognac.
Stir the products to obtain a homogeneous mixture. Make small incisions in the shanks with a knife, then distribute the marinade in them and all over the entire surface.
Transfer the meat to a baking bag, along with the thyme and rosemary sprigs, then place in the fridge for at least 12 hours. Place the bag with the marinated shanks in a tray in which you've poured 1/2 cup water.
Poke several holes in the bag with a toothpick and put to bake in a preheated 338°F (170 °C) oven for 2 1/2 hours. Shortly before they're ready, remove the bag and finish baking the shanks to a nice, appetizing crust.
Serve with a garnish of your choice. What I did was I arranged several seasoned potatoes around the shanks halfway through baking since I have a large tray.