Remove the leaves from a head of Napa cabbage (except the smallest middle leaves - you’ll use about 18 leaves from a 2 pound cabbage). Bring a large pot of water to a boil. Add the cabbage leaves and boil them until softened, about 5 minutes. Drain, pat dry, and set aside.
Heat 2 teaspoons of the olive oil in a skillet over medium heat, and add the leeks and broth. Cover, bring to a simmer, and reduce heat to medium-low. Cook until the leeks are tender, about 10 minutes. Drain the liquid and set the leeks aside.
While the leeks are cooking, place the tomatoes in another skillet, and stir in the salt and maple syrup. Heat the mixture over medium high heat for about 5 minutes, stirring periodically, until most of the liquid is gone.
Preheat the oven to 350 degrees and coat a 9 x 13 inch pan with the remaining teaspoon of olive oil.
Layer the food in the 9 x 13 pan as follows: place half the cabbage in the baking dish, followed by half the tomato sauce, half the leeks, and half the sliced mozzarella. Repeat with a second layer in the same order, saving a bit of tomato sauce to sprinkle on top.
Bake the casserole for 25 minutes, or until it begins to turn brown at the edges and the cheese has melted. Garnish with pine nuts or walnuts and fresh herbs, if desired. Serve warm.