In a large heavy based saucepan melt the butter with the olive oil. Reduce the heat and add the onions, salt and pepper and herbs. Stir gently, cover and on low heat cook for 20 minutes, stirring every now and then, approximately in every 4-5 minutes. Try not to brown the onions. Remove the lid and cook uncovered for a further 15 minutes, stirring frequently until the onions have caramelized. In the mean time heat a small drizzle of olive oil in a medium sized non-stick pan. Add the potatoes and cook for 2-3 minutes then reduce heat and continue cooking, turning frequently and making sure the potatoes do not brown much. If you cook new potatoes, about 15 minutes should be enough. Once ready, remove from pan and set aside. In a large bowl lightly beat the eggs with a pinch of salt. Add the potatoes and onions, and stir to coat. If you are going to add green pepper, as we did, do it now. Turn the heat to medium and add the mixture in the pan. With a wooden spoon give the mixture in the pan a good stir. Once the eggs start to cook leave the mixture to set, simply evening the top. Cook for 2-4 minutes on this side. With a help of a plate flip the tortilla and cook for another 2-4 minutes on this side. Depending on how you like your eggs cook it for more or less. Serve warm.