Heat 30ml oil in a casserole pan and fry onions, courgettes, celeriac, and garlic for 5 minutes.
Add tomato purée and smoked paprika and cook for a further 5 minutes.
Stir in chopped tomatoes and stock, bring to boil, then simmer for 25 minutes.
Stir in butter beans, basil, and lemon zest and cook for 10 minutes.
Stir in broad beans and finish with extra-virgin olive oil.
Mix mashed potatoes, baking powder, flour, salt, pepper, and thyme in a bowl.
Shape the potato mixture into six patties.
Heat oil in a frying pan and fry patties for 4 minutes per side until golden.
Serve the stew in bowls with potato cakes on the side.