To make the crumble, combine the crackers, flour, cubed butter and the cheese in a food processor and process only in very brief pulses until the mixture resembles a very coarse sand texture. Try to keep the mixture as loose and fluffy as possible and reserve in a cool area until needed. Preheat your oven to 375 degrees, with the All-Clad Oval Roaster set in the center of the oven. Separate the bacon strips and evenly distribute them in the hot pan. Bake the bacon for fifteen minutes then stir in the leeks and return to the oven for another fifteen minutes. Add the yams and the 1 Tablespoon plus 1 teaspoon of kosher salt, stir well and bake for another half hour, stirring occasionally. While the yams are baking, heat a 4 quart All-Clad sauce pot over a medium flame. Add the butter to the pan and when it melts add the flour and continue to cook for a couple minutes. Slowly pour the milk into the pot in a slow stream while stirring or whisking vigorously. It is best to add the milk a little bit at a time and cook until it begins to thicken. Continue to cook and stir until the milk is completely thickened then turn the heat down, cover the sauce pot with a lid and continue to cook for another 15 minutes stirring occasionally. Stir in the cheddar cheese little by little until it is melted and fully incorporated then remove the pot from the heat. Stir in the minced thyme and season the sauce with the 2 teaspoons of kosher salt. Keep the sauce hot and covered until the sweet potato is tender. Pour the sauce into the All-Clad Oval Roaster, stir to mix the ingredients then gently flatten level the surface with a spoon or rubber spatula. Sprinkle the crumble evenly over the surface of the gratin. Set the gratin in the center of the oven and bake for 60 minutes. The gratin should be golden brown and bubbly. Check the doneness of the yams by piercing them with a paring knife, there should be no resistance.