Bring a large pot of water to the boil. Add the pasta to the water and cook as per packet instructions. Add the broccoli into the pasta water to cook for the last 4mins. Drain pasta and broccoli and set aside.

Meanwhile, heat oil in another saucepan on medium heat. Sauté the onion and garlic in the pan for 2min until it starts to soften. Then add the butter to the pan, when melted stir through the flour until there are not flour lumps. Reduce the pan to low heat and gradually whisk in the milk. Bring to a gentle simmer until sauce starts to thicken. Stir through the cheddar and wholegrain mustard. Season with salt and pepper. Finally add the pasta and broccoli into the sauce and stir to coat.