Heat the oil in an 11-inch skillet or frying pan over low heat. Add the onion and pan-fry, stirring occasionally, for 4 minutes, then stir in the bell peppers, eggplant and zucchini.
When everything is browned add the tomatoes, and let cook until the water has evaporated and the mixture has thickened. Beat the eggs vigorously with a pinch of salt in a large bowl. Increase the heat under the vegetables. Tip in the egg mixture and cook, gently shaking the skillet occasionally, until the underside is set and lightly browned. Invert the tortilla onto a lid of a pan or a plate, then gently slide it back into the skillet, cooked side up. Cook, gently shaking the skillet occasionally, until the underside is set and golden brown. Serve immediately or at room temperature, cut into wedges.
Note: If you have any of this left over, it is tasty served cold with a little mayonnaise spread across the top.
Shared with permission from The Book of Tapas by Simone and Inés Ortega, published by Phaidon Press, 2010 click for book review