In saucepan, combine potatoes with cold, salted water to cover. Bring to a boil, reduce to a simmer, cover, and cook until potatoes are tender, about 15 minutes. Drain potatoes and let air-dry. When cool enough to handle, thinly slice.
Meanwhile, in large nonstick skillet, heat olive oil. Add onion, reduce heat, and cook until slices begin wilting and turn translucent, about 10 minutes. Add potatoes to skillet. Gently stir to combine and cook until potatoes are soft and cooked, but not browned, 5 to 10 minutes.
In a large bowl, combine egg substitute, milk, and salt and pepper to taste. Gently add potato mixture.
Wipe out nonstick skillet, coat with cooking spray, and place over heat. Add potato-egg mixture and reduce heat to low. Cover and cook for 10 minutes, or until bottom of tortilla is golden and top begins to set.
To flip, cover skillet with large plate; place one hand on plate to keep in place and invert skillet. Return skillet to stove, coat with more cooking spray if necessary, and slide tortilla back into skillet. Reduce heat to low. Continue cooking until tortilla browns on second side and center is firm. Slice and serve warm or at room temperature.