Peel and finely chop the onion. Trim and dice the courgette and aubergine, then deseed and dice the pepper. Core and dice the tomato.
Heat the oil in a large pan on a medium heat, add the onion and fry for 5 minutes, or until softened.
Add the courgette, aubergine, pepper and tomato, then pour in 120ml water and add the tomato purée. Bring to the boil, then reduce to a low heat and simmer for 20 minutes, or until the vegetables are tender and sauce has thickened, stirring occasionally.
Peel and dice the potato. Cook in boiling water for 15 minutes, or until cooked through, then drain and mash with the milk.
Chop the ratatouille as required, then allow both the mash and the ratatouille to cool before serving. For more information on stages of complementary feeding, click here.