Bring a large pot of salted water to a boil. Add the pasta and cook for 7-10 minutes. The pasta should be quite firm still as it will continue cooking in the oven. Add the broccoli florets and cook for the last 2 minutes of cooking. Reserve 1 cup of cooking water and drain.
While the pasta is cooking, melt the butter in a deep pan or saucepan then whisk in the flour.
Cook for 1-2 minutes then whisk in the garlic powder and thyme.
Add the milk, stirring/whisking continuously, until smooth. Cook over medium heat until the sauce has thickened and the flour has cooked through.
Stir in the cream, grated cheese and season with salt and pepper.
Combine the pasta and broccoli with the cheese sauce. Add a little pasta water if the sauce is too thick.
Transfer to a baking dish and top with grated cheddar cheese. Place in a preheated oven and bake for 15-20 minutes until golden brown and bubbling.
Remove from the oven, allow to rest for 5 minutes then serve.