Broccoli Gratin with Mustard-Cheese Streusel

In a large nonstick skillet, melt the butter over medium-high heat. Add the onion and garlic; sauté until the onion is soft, about 4 minutes. Stir in the dry mustard, then 2 tablespoons of the Dijon, blending well. Add the breadcrumbs; sauté until the crumbs are crisp and golden, stirring and pressing with the back of a fork to separate, about 8 minutes. Season with salt and pepper; cool completely. Mix in 3/4 cup of the cheese.
Cook the broccoli florets in large pot of boiling and generously salted water until crisp-tender, about 3 minutes. Drain. Transfer to paper towels; cool. (Streusel & broccoli can be made 1 day ahead. Cover separately and refrigerate.) It's important that you don't over-cook the broccoli- you don't want mushy broccoli!
Preheat the oven to 350°F. Butter a 13x9-inch glass baking dish. In a small bowl, combine the whipping cream, remaining 2 tablespoons Dijon mustard, and 1/4 cup cheese. Place the cooked broccoli in a large bowl, and add the cream mixture; toss to coat evenly. Arrange the broccoli in the prepared dish. Sprinkle the breadcrumb streusel over the broccoli. Bake until the broccoli is heated through and the streusel is browned, about 30 minutes.