Heat 6 tablespoons of olive oil in a thick bottomed wide pan with a lid (I use a round Le Creuset pan) over medium heat.
Add the onions and sauté them until they are soft. Sprinkle with a little salt to prevent browning.
Add the eggplant and cook for a minute or so.
Add the zucchini, followed by the peppers.
Grate in the garlic.
If the vegetables absorb all the oil and you feel you need more, feel free to add it as you go along.
Cover the pan, turn the heat back and cook gently over very low heat for 30 minutes, stirring occasionally, until the vegetables have all softened a little.
Add the tomatoes or passata and herbs and season to taste with salt and pepper.
Stir the tomatoes or passata through the vegetables. Cover the pan and cook over low heat, stirring occasionally, for another 30 to 40 minutes until everything has come together. The vegetables should be soft but not mushy.
Garnish with basil or parsley.