Heat large skillet over medium heat. Add 1 tablespoon butter. When melted, add sliced onion, marjoram sprigs.pinch of sea salt and black pepper. Cook, stirring frequently, for 5 minutes. Turn heat to low. Cook, stirring occasionally, for 25 minutes, until onions caramelize.
Meanwhile, preheat oven to 400°F. Butter deep 9 x 13-inch baking dish and set aside.
Bring large pot of salted water to boil. Blanche asparagus spears for 1 1/2 minutes. Remove from water with tongs, immerse spears in ice water and set aside. Add pasta to boiling water and cook to al dente. Drain and set aside.
Meanwhile, drain and chop cooked asparagus into 1 – 2-inch segments.
Heat remaining 4 tablespoons butter in large saucepan over medium heat. Whisk in flour and pinch of sea salt. Cook 2 minutes. Gradually whisk in milk. Cook, stirring often, until mixture thickens, about 5 minutes. Turn heat to low and gradually stir in fontina, reserving 2 tablespoons.
Add cheese sauce to cooked pasta and fold in caramelized onions and chopped asparagus.. Spoon into prepared baking dish. Top with reserved 2 tablespoons fontina.
Bake 35 - 40 minutes or until filling is bubbling and top is golden brown.
Garnish with reserved teaspoon minced marjoram. Serve immediately.