Melt butter in a large non-stick skillet over medium heat. Add onion and garlic; saute until onion is soft, about 4 minutes. Mix in dry mustard, then 2 T Dijon mustard, blending well. Add breadcrumbs; sauté until crumbs are crisp and golden, stirring and pressing with back of fork to separate, about 8 minutes. Season with salt and pepper; cool completely. Mix in 3/4C cheese.
Cook broccoli florets in large pot of boiling generously salted water until crisp-tender, about 3 minutes. Drain. Transfer to paper towels; cool. (Streusel and broccoli can be made one day ahead. Cover separately and refrigerate).
Preheat oven to 350°F Butter 13x9x2-inch glass baking dish. Place broccoli in large bowl. Add whipping cream, remaining 2 T Dijon mustard, and 1/4 cup cheese; toss to coat evenly. Arrange broccoli in prepared dish. Sprinkle streusel over. Bake until broccoli is heated through and streusel is browned, about 30 minutes.