For the broth: Preheat the oven to broil. Place the onion and ginger on a baking sheet and broil until blackened on top, about 10 minutes.
Cook the bacon in a large pot over low heat until all of the fat is rendered out and the bacon is crisp, about 10 minutes. Put the roasted onion and ginger, the garlic, green onions, leek, broth, soy sauce, mirin and salt into the pot. Simmer for 2 to 3 hours, while you cook your pork belly. Strain to serve.
For the pork belly: Preheat the oven to 350 degrees F. Sprinkle the belly with the salt, sugar and pepper. Throw the seasoned belly in a roasting pan and pour the sake over it. Roast for 1 hour 30 minutes, and then bring the heat up to 450 degrees F and let it caramelize. It should be tender but not mushy.
Let the belly cool to room temperature. Wrap it up tight in plastic and put it in the fridge until it¿s thoroughly chilled through. At that point, slice it into nice, thick slabs. Set aside for ramen.
To assemble the ramen: Cook the ramen noodles according to package instructions. Place the noodles into a bowl, followed by pork belly and top with broth. Add whatever toppings you like!