Preheat oven to 180℃/375℉.
Bring a large pot of water to a boil. Add the pasta and cook according to the package directions, reducing the time by one minute for al dente. In the final minute of cooking, add the broccoli. Once cooked, drain everything in a colander and set aside.
While the pasta is cooking, combine the milk and spinach in a blender and blend until the spinach is fully puréed and the milk turns green.
Melt butter in a large saucepan, over medium heat. Add the flour and cook, stirring continuously for 1-2 minutes.
Add the spinach and milk mixture, a little at a time, and keep stirring/whisking continuously between each pour. Stir out any lumps before adding more milk. Continue adding the milk little by little until all the milk is incorporated and the sauce is thick and glossy. When the sauce clings to the back of a wooden spoon, you should be able to trace a line with your finger.
Take the sauce off the heat and stir in the mustard and grated cheese. Taste the sauce and season to taste. You may find you need less or more salt depending on the cheese used.
Pour the sauce into the pot with the drained macaroni and broccoli. Add the peas and stir. Pour into a 9 x 13″ / 23 x 33 baking dish and top with cheese.
Bake for 15-20 minutes or until top is lightly golden and bubbly. Be careful not to overbake, or you might dry out the sauce