Preparation
1. Preheat oven to 350 F. Spray a 9x13 in dish with nonstick spray.
2. Bring a large pot of water to boil. Add pasta and cook 5 minutes. Add broccoli and cook, stirring occasionally, until pasta and broccoli are tender, about 3-4 minutes. Drain; return the pasta and the broccoli to the pot.
3. While the pasta and broccoli are cooking, heat oil in a large saucepan over medium heat. Add the onion and cook, stirring occasionally, until tender, about 5 minutes. Add the flour and mustard; cook, stiring constantly, about 1 minute. Gradually whisk in the milk and salt. Increase the heat and cook, whisking constantly, until the sauce boils and thickens, 3-4 minutes.
4. Add the sauce the the pasta and broccoli; stir to combine. Transfer to the prepared baking dish and sprinkle evenly with the cheese. Bake until bubbling, about 25 minutes.