Heat your oven to 180°C/Fan 160°C/Gas 4. Put a large pan of salted water on to boil. Break the broccoli into small florets (you can keep the stalk and use it sliced in stir-fries or to make soup). When the water is boiling, add the pasta and broccoli. Simmer for 5 mins, then drain.
While the pasta simmers, pour the milk into a separate pan. Add the bay leaf. Set over a medium heat and slowly bring to the boil, whisking occasionally to stop the milk catching and burning. Keep your eye on the pan as milk can boil over very quickly, so you may need to swiftly turn the heat down if it starts to foam up.
While the milk heats up, pop the butter and 2 tbsp flour in a bowl. Mash them together till combined. When the milk comes to the boil, whisk the butter and flour in, tsp by tsp, till it's all combined. Simmer the milk for 8-10 mins, whisking, till it starts to thicken. Take the pan off the heat. Fish out the bay leaf.
Grate the cheddar and stir ¾ of it into the milk. Grate in ¼ of the nutmeg. Taste the sauce and add salt and pepper if you think it needs it.
Tip the pasta and broccoli into an ovenproof dish. Pour over the cheese sauce. Scatter over the breadcrumbs and tip in the rest of the grated cheddar. Slide into the oven and bake for 30 mins till golden and bubbling.
While the pasta bake cooks, toss the salad leaves with 1 tbsp balsamic vinegar. When the bake is golden, spoon into onto warm plates and serve with the salad.