Slice potatoes and onion into ¼-inch thick pieces.
Heat oil in a large pot over medium. Add potatoes, cover, and simmer for 5 minutes. Stir, then add onions and salt. Cover and cook for 10 minutes until soft. Avoid boiling the oil. Remove with a slotted spoon, cool on a tray, and save the oil. Adjust salt if needed.
Transfer cooled potatoes and onions to a bowl. Shake and pour in Just Egg, then mix well.
Heat 2 tbsp of reserved oil in a 10-inch nonstick skillet over medium. Add mixture, distribute evenly, cover, and cook for 10 minutes. Check halfway to prevent burning. The bottom should be golden. Remove from heat.
Slide onto a plate, place another plate on top, and flip. Add 1 tbsp oil to the skillet, then slide the tortilla back in with the uncooked side down. Shape edges for a round tortilla.
Cover and cook for 5 minutes until set. A knife inserted should come out clean. Transfer to a plate and let it sit for 10 minutes.
Slice and serve on baguette slices.