Cook pasta according to package directions for al dente consistency. Drain and return the pasta to the cooking pot. Add olive oil, salt, and black pepper; toss to coat. Adjust seasoning to taste and transfer pasta to a 9x13-inch baking dish.
Heat a medium-size skillet over medium-high heat, then add 1 tablespoon of butter. When the butter is melted, add onion and asparagus. Cook for 3 minutes, until onion is softened and asparagus is bright green in color. Add the garlic and continue cooking for an additional 30 seconds. Layer the asparagus mixture over the cooked pasta.
Sprinkle mozzarella cheese over the asparagus layer.
In a small bowl, use the microwave to melt the remaining 3 tablespoons of butter. Combine the melted butter with the Panko breadcrumbs, then sprinkle over the cheese layer.
Cover and bake for 20 minutes. Uncover and continue baking for an additional 10 minutes, until the breadcrumbs are lightly golden. Sprinkle with Italian parsley and serve hot.