Leek and broccoli barley bake

This is normally a variation on macaroni cheese; however, as I had cooked pearl barley from the lemon-barley water I'd made earlier, I thought I would see whether using barley instead of pasta would work - and it did! This variation also had mushrooms and tomatoes!

1/2 cup (125 ml) pearl barley, uncooked.
1 litre water
c 20 g butter
1/2 pack button mushrooms, sliced
1 leek, chopped
1/2 head broccoli, divided into florets and the stem chopped
2 tomatoes, thinly sliced
250 ml milk
1.5 tsp plain flour
(Allegedly) 60 g grated cheese (Cheddar and/or Emmenthal - but I think I used rather more than that, and it may even have been as much as 60g of each!)

Cook the barley in plenty of water, which you might then use to make lemon barley water. Or not. In either event, put the barley in an oven-proof dish.

Meanwhile, steam the broccoli, and cook the mushrooms and leeks in the butter. Whisk the flour and milk together with seasoning (salt, pepper and 1/4 tsp dry mustard will do), and pour on to the mushrooms/leeks. Stirring constantly, bring to the boil and allow to thicken. Turn off the heat and add half the cheese. Put the broccoli in with the barley and pour the sauce on the top. Stir until thoroughly mixed. Top with the sliced tomatoes and the rest of the grated cheese.

Bake in a moderately hot oven for 45 minutes.