Place the potatoes and onions in a large pot, and pour in 4 cups of the olive oil, adding more if necessary to cover. Add in the bay leaves and garlic and simmer over medium heat, stirring occasionally until the potatoes are tender enough to be smashed by a spoon, but not falling apart, 15 to 20 minutes. Remove from the heat. Carefully drain the oil and cool. Reserve the oil for the next time you make a tortilla, it will continue to develop flavor and make your tortillas better. Cool the potatoes and onions.
Preheat the oven to 350 degrees F.
Whisk the eggs with a pinch each of salt and pepper, until combined. Carefully fold in the onions and potatoes. Set aside.
In a large cast-iron skillet, heat the remaining 2 tablespoons oil over medium heat. Pour the egg, potato and onion mixture into the pan, shaking to evenly distribute. Carefully stir until level and then cook just until it begins to set, about 5 minutes. Transfer to the preheated oven and cook until set, 20 to 25 minutes.
Here comes the fun part. Remove from the oven, and place a large cutting board or plate over the skillet. Carefully invert the skillet to transfer the tortilla onto the board. Gently slide the tortilla back into the pan and return to the oven for another 10 minutes. Remove from the oven and immediately remove from the pan. Cool the tortilla 20 minutes before slicing and serving.