Heat the oil in a frying pan and add the shallots, garlic and potato slices. Cook on a low heat for about 10 minutes, until the potatoes become a little crispy and the onions have softened.
Whisk the eggs in a bowl and season.
Add the eggs to the potatoes and cook on a gentle heat for around 8 minutes.
Put a plate on top of the frying pan (big enough so it covers it) and then carefully flip the pan upside down whilst holding the plate and then slide the tortilla back in to the frying pan so it is now cooking the other side. Cook for another 5-6 minutes, so the eggs are cooked through.
Keep this well wrapped up and it will keep in the fridge for up to three days. Cooked egg dishes freeze well, so this tortilla can be kept in the freezer for up to one year, although they are of best quality up to six months.