Heat the oven to 180°C. Rub the peppers with olive oil and roast in the oven until the skin is charred. Place in a bowl while still hot and cover tightly with cling film. Leave to cool.
Meanwhile dice the onion and sauté without colouring for approx. 10 minutes. Add all but 3 tomatoes and season well. Bring to a simmer and stir occasionally breaking up the tomatoes gently with the spoon as it cooks.
When the peppers are cooled carefully rub off the charred skins, removing the seeds and cut into 3 pieces. Slice the aubergine into 1cm slices and with a cutter cut a round/ oval out of the 3 different peppers and the aubergine. Leave to one side.
Dice the half remaining tomatoes, peppers and all of the aubergine and sauté.
Blend the tomato sauce until smooth and remove a third of it to a clean pan. Add the sautéed vegetables to the rest of the tomato sauce and leave on a low heat.
Finely chop the remaining vegetables and add to 2 tablespoons of the tomato sauce. Simmer for 10 minutes then finely blend. Use this mixture as the glue in the layers of peppers.
Char grill the aubergine round on both sides prior to stacking with the peppers, using the blended sauce to keep together. Place in the oven for 8 minutes.
Transfer to a warm plate. Drizzle the tomato sauce around the outside and also the ratatouille.