Combine oil, onion and potatoes in a 10-inch non-stick skillet. Oil should cover vegetables. Heat over medium-low heat.  Cover; Oil should bubble slowly, about 12 minutes or until potatoes are tender.

Transfer to vegetables to a large strainer over a large glass bowl.  Reserve the oil. Let the vegetables cool for 10 minutes.

Place eggs in a large bowl, season with salt and pepper, whisk to mix well. Fold in vegetables and set aside for 10 minutes.

Heat 2 tablespoons of reserved oil in 10-inch non-stick skillet over medium heat.  Pour in egg and vegetable mixture and cook until the edges begin to set.  Run a silicone spatula around the edges to separate the egg from the skillet. Continue running the spatula until bottom portion is completely set and separated from the skillet. Place a large plate over the skillet, flip the tortilla onto the plate.  Place skillet back on stove then slide tortilla (uncooked side down) onto skillet.  Cook until the bottom is set, pushing any uncooked egg down the edges with spatula ; about 3 minutes. 

Place large plate over skillet and invert, flipping the tortilla onto the plate.  Let rest at room temperature for 10 minutes.  Slice into wedges and serve with a side of sour cream and a  sprinkle of fresh thyme, parsley, and chopped tomatoes.