Spray a 1-quart gratin pan or baking dish with nonstick spray and set aside. Set the oven rack high in the oven (highest rung) and preheat the oven to 350°F.
Heat the milk until it is very hot (fine to do this directly in the measuring cup in a microwave, if the cup is microwave proof.) Do not let the milk boil. When it is hot, set aside near the stove.
Put a pot of lightly salted water to boil. When it reaches a boil, add the macaroni and let it cook until tender. When it is tender, toss in the chopped broccoli and wait about 30 seconds, then drain the entire potful into a colander in a sink.
While the macaroni is cooking, place a medium-large saucepan over medium heat. Add the oil, and use a narrow, closely-tined whisk to beat in the flour, mustard, and garlic powder. Keep whisking until it forms a thick paste (this will happen within seconds!) and then slowly drizzle in the hot milk, still vigorously whisking, so it becomes smooth as the milk is incorporated. Turn the heat to low, and switch to mixing with a wooden spoon. Keep cooking and stirring until it is velvety, thick, and smooth.
Add the salt, half the cheddar, the Parmesan, and the cayenne, and stir until the cheese is fully blended. Remove from heat. Add the cooked macaroni and broccoli to the cheese sauce, also adding the remaining cheddar. Stir until all the pasta is well coated. Transfer to the prepared pan, top with the bread crumbs and walnuts, and bake uncovered in the high part of the oven for about 20 minutes, or until bubbly around the edges and crisp on top. Serve warm.