Preheat the slow cooker if necessary; see the manufacturer's instructions. Heat the oil in a frying pan, add the onion and aubergine and fry, stirring, for 5 minutes or until softened and just beginning to turn golden.
Stir in the courgettes, peppers and garlic and fry for 3–4 minutes. Mix in the flour, then the tomatoes, stock, rosemary and a little salt and pepper. Bring to the boil, then spoon into the slow cooker pot. Cover and cook on high for 3–4 hours until the vegetables are tender.
When almost ready to serve, make the dumplings. Put the flour, ricotta, lemon rind and a little salt and pepper into a bowl. Add the egg and mix to a soft but not sticky dough. Cut into 12 pieces and roll each piece into a ball with floured hands.
Stir the ratatouille and arrange the dumplings on the top. Replace the lid and cook for 15–20 minutes or until light and firm to the touch. Spoon into bowls and eat with a spoon and fork.