Finish the Sauce: Reduce the heat to low. Stir in the shredded cheddar, Parmesan, garlic powder, onion powder, salt, and pepper. Whisk until the cheese is melted and the sauce is smooth. Remove from heat.
Assemble the Casserole: Spread the blanched asparagus and diced onion evenly in the prepared baking dish. Pour the warm cheese sauce over the top, ensuring it coats the asparagus.
Add the Topping: In a small bowl, combine the panko breadcrumbs, crushed Ritz crackers, and melted butter. Mix until the crumbs are evenly moistened. Sprinkle the crumb mixture evenly over the casserole.
Bake: Bake, uncovered, for 20-25 minutes, until the casserole is bubbly around the edges and the topping is golden brown and crisp. For an even browner top, you can switch the oven to broil for the final 1-2 minutes, watching it carefully to prevent burning.
Rest & Serve: Let the casserole rest for 5-10 minutes before serving. This allows the sauce to set slightly, making it easier to serve.
