Peel and dice the onion. Peel and roughly chop the garlic. Drain the cannellinni beans into a colander. Rinse well. Crumble the stock cube into a measuring jug. Add 225 ml boiling water. Stir.
Heat a heavy-based pan. Add the onion with 2 tbsp cold water. Season. Cook on a low heat for 5 mins. Stir now and then till the onion looks soft.
While the onion cooks, halve the chilli. Flick out the white bits and seeds for a milder flavour. Finely chop it. Roughly chop the chervil sprigs.
Add the garlic to the onions. Cook and stir for 1 min. Reduce the heat. Measure out 1 tbsp tomato purée. Add it with 1 bay leaf, the cannellini beans, thyme sprigs, 1 tsp of the sumach and the stock.
Pop a lid on the beans. Simmer for 5-10 mins to get a thick stew. Stir now and then. Taste and adjust the seasoning. Fish out the thyme sprigs.
While the beans cook, wipe the mushrooms clean. Heat your grill to high. Brush the mushrooms with ¼ tbsp olive oil each. Grill them, smooth side up, for 3-4 mins till golden. Turn them over and grill for another 2-3 mins till browned and juicy.
When the beans are ready, serve them heaped on the grilled mushrooms. Top each serving with half the crumbled feta, the chilli and the chervil.