Cook the lasagna al dente according to the package directions, about 8 -12 minutes. Place the cooked pasta in an ice water bath to stop the cooking. Drain and lay out on a baking sheet and cover.
In a small bowl, mix the ricotta and Parmesan cheeses; add salt and pepper and set aside.
Heat 2 Tbsp. olive oil over medium heat in a 3-qt. sauté pan. Add the leeks and sauté until they are very soft and light golden brown, about 3 minutes. Remove from the pan and reserve.
Heat the remaining olive oil in the sauté pan. Add the mushrooms and sauté until they are soft and the liquid has evaporated, about 7 minutes. Add the garlic and cook for a minute. Return the leeks to the pan and heat through.
Melt the butter in a 3.5-qt. sauce pan over medium heat. Add the flour stirring constantly for 2 - 3 minutes. Add the milk and cook, stirring constantly, until the liquid returns to a boil and thickens. Add the gorgonzola, salt, pepper and nutmeg and stir until smooth.
Preheat the oven to 375°F/190°C. Grease a 9x13-in. nonstick baking pan. Place a thin layer of sauce in the bottom of the pan. Cover the bottom of the pan with a single layer of pasta. Add 1/3 of the sauce mixture over the pasta followed with 1/3 of the mushroom/leek mixture over the sauce. Add a layer of noodles. Repeat 2 more times. Sprinkle the mozzarella over the top layer and bake until the top is golden and bubbling around the edges, about 40 - 50 minutes.