1. Preheat oven to 350 degrees Fahrenheit.
2. Slice the potatoes and onion into thin slices about the thickness of two pennies with a knife or mandoline. Meanwhile, heat the oil in a 10-inch nonstick pan (with a lid) over a medium flame. After a minute or two, place a few pieces of potato in the oil and check to see if small bubbles are forming around its side. Once you see the bubbles, place all the potatoes and onions in the hot oil and stir to coat. Lightly season with salt and pepper and continue to cook over medium heat, stirring and flipping the potatoes and onions occasionally. When the potatoes are knife-tender, strain them and the onions with a colander over a heatproof bowl. Reserve the oil.
3. In a separate bowl, crack and whisk the six eggs and lightly season with salt and pepper. Place the pan back on the stove over a medium-high flame and add two to three tablespoons of the reserved oil. Mix the strained onions and potatoes into the egg. Pour the potato-egg mixture into the hot pan. Cook for one minute, then place a lid over the pan and place into the preheated oven for around 10 to 15 minutes. You will know the frittata is done when the top is set and no longer runny. Remove from the oven.
4. To plate, carefully place a large plate over the pan and invert the plate with the pan, leaving the upside-down frittata on the plate. Cut into wedges and serve. The frittata can be served warm or cold.