1. Cook the potatoes in salted boiling water for 10-15 minutes, until tender. Drain, then push through a sieve.
2. Preheat the oven to 200C/400F/Gas 6.
3. Cook the roughly chopped fennel in salted boiling water for about 7 minutes, until tender, then drain.
4. Heat the oil in a frying pan, add the onions and fry for 5-10 minutes. Add the garlic and cook for a further 1 minute. Add the mushrooms and cook for a further 2 minutes. Season.
5. Sprinkle half the breadcrumbs over the base of a shallow ovenproof dish. Add half the mushrooms and onions, fennel, parsley, basil, mozzarella and cheddar cheeses and potatoes in layers. Repeat these layers, finishing with a layer of potatoes, and sprinkling over the remaining breadcrumbs.
6. Bake for 20 minutes until golden and bubbling. Serve straight from the oven with a crisp green salad.