In a large, heavy-bottomed (preferably non-stick) pot, heat the olive oil and vegetable oil until hot but not smoking. Add the potato slices, onion and about ½ teaspoon salt. Lift and turn the mixture frequently. The potatoes will soften as they boil in the oil but should not brown. Cook until the potatoes are soft (not mushy) and are easily cut with a knife.
While the potatoes cook, crack the eggs into a large bowl and beat with ½ teaspoon salt until frothy.
When potatoes are soft, remove the mixture with a slotted spoon to drain the oil and add to the beaten egg. Gently push down the mixture until it is submerged and let rest 15 minutes.
Before cooking the tortilla in the skillet, make sure that you have a plate that is larger than the skillet! It may be easier to make 2 smaller tortillas than one large one. You will also need a lid for the skillet.
In a non-stick (very important to use non-stick!) skillet, heat 1Tablespoon oil (you can use the leftover oil from the pot) until quite hot. Slide in the egg/potato mixture and push down to fill the skillet. Shake the pan, lower the heat to medium high and cover with a lid. Shake the pan to prevent sticking. When it starts to brown on the bottom (lift gently with fork to check), cover the skillet with a plate and flip the tortilla onto the plate (first flip is best done over the sink). Slide tortilla back into skillet and cook on the other side in the same manner. Lower heat to medium and continue to flip 4-5 more times until tortilla is cooked through – 15-20 minutes.
Before serving, I press a paper towel onto both sides to absorb excess oil. Serve with sliced baguette.