It was Regather’s prize-winning flower grower/wrangler/designer, Rachel, who recommended this dish the other day. Mushrooms, onions and winter roots in a lovely mustardy gravy and a crispy topping.  That’s a Goldilocks dish: not too heavy, not too light. Rachel tells me the recipe doesn’t suffer for having less than the full whack of mushrooms – use what you have.  And we …