Heat a medium-sized pan with 1/2 tbsp oil and add the garlic, mushrooms and kale and cook for 5 minutes until softened.
Meanwhile, dissolve the stock cube in a jug of 50ml boiling water.
Add the drained butter beans and stock to the same pan as the vegetables, along with the chestnuts, sun-dried tomatoes, the leaves from 1 sprig of thyme and the passata. Mix well and leave to simmer for 15 minutes until the sauce has thickened.
Place sweet potatoes in a pan of boiling water with a pinch of sea salt and boil for 10 minutes until soft.
Preheat the grill to high. Drain and mash the sweet potato with a potato masher or the back of a fork. Stir in the leaves from 1 sprig of thyme.
Place the vegetable and bean mix in a small ovenproof dish and top with the sweet potato mash. Grill for 5 minutes until golden brown.