Cover the bottom of a frying pan in olive oil (use extra virgin if you're able) and heat on medium
Add all of the cut vegetables and cook at a slow simmer for about 10 minutes or until the potatoes are fully cooked through (but not breaking apart)
In the meantime, whisk six eggs and add the grated Manchego cheese and a dash of double cream if you wish (note: this is not traditional to Spanish tortilla)
Season with salt and pepper to taste
Remove the vegetables from the frying pan with a slotted spoon and strain away any excess oil
Fold the vegetables into the egg mixture gently - try not to break the potatoes
Add the mixture to a frying pan that's coated with a couple of tablespoons of olive oil
Cook on medium heat for about seven to ten minutes -- the bottom should be golden brown and the middle should be starting to set
Cover the frying pan with a large plate and quickly flip the tortilla onto the plate so that the raw side is down
At this point, you can choose if you want to place it into a smaller frying pan. I learned this tip from a proper Catalan, who said always move your tortilla into a smaller frying pan to get that nice rounded shape.
Slide the tortilla back into the pan to cook the other side
Cook for another five to ten minutes, depending on how well done you like your tortilla
Flip back onto a plate and let rest at least 10 minutes (this is important so that the tortilla sets)