In a oven-proof casserole dish, add potatoes and toss with a generous amount of olive oil (these are going to "poach" in the oven). Cover and cook for 20 min.
In a medium sized saute pan, add some olive oil and onions. Cook until translucent and "sweet" (don't caramelize). Remove from stove and add to potatoes, re-cover and cook another 20 minutes. Remove and cool to room temp. Fold in egg mixture and season with salt.