Remove the leaves from the chard and trim the stalks. Bring a pan of salted water to the boil, add the lemon juice then blanch the stalks until tender. Drain and set aside.
Heat the olive oil in a frying pan and saute the mushrooms until golden. Add the garlic, then the chard stalks and finally the leaves, Stir everything well, remove from heat and set aside.
Preheat the oven to 180 C, Fan 160 C, Gas 4.
Melt the butter in a small pan, add the flour and cook for 2 minutes. Slowly add the milk and whisk to make a thick sauce. Season with salt and pepper then add the cheese and allow to melt.
Mix the sauce with the chard and mushrooms then pour into a buttered ovenproof dish. Sprinkle over the grated parmesan and breadcrumbs.
Bake in the preheated oven for 20 minutes until heated through and golden brown on top.