Steaming instead of boiling the broccoli prevents it from getting waterlogged and thinning the creamy cheese sauce.
Preheat the oven to 450°. Place a steamer basket in a large pot with a lid; add 1 inch of water and bring to a boil. Add the broccoli. Cover and steam until crisp-tender, about 5 minutes. Transfer to a baking sheet lined with a kitchen towel and pat dry.
In a medium skillet, melt 2 tbsp. butter. Add the panko and toss to coat. Transfer to a plate; mix in the Parmesan. In a medium saucepan, melt the remaining 3 tbsp. butter over medium heat. Add the onion and cook, stirring occasionally, until soft, 3 to 4 minutes. Add the flour and stir until golden brown, about 3 minutes. Add the milk and cook, stirring frequently, until the sauce thickens, 3 to 5 minutes. Add the Gruyère and mustard and stir until the cheese melts, about 1 minute. Add the broccoli to the sauce; fold to coat. Transfer to a 1 1/2-quart baking dish and top with the Parmesan panko. Bake until browned in spots and bubbling at the edges, about 20 minutes. Let stand for 5 minutes before serving.
