Slice 3 small red potatoes & 1 small red onion into very thin circles (I use a mandoline).
Using a 9-in (23-cm) (preferably nonstick) skillet, fry the potatoes & onions with salt, pepper & 3 T olive oil on low heat for about 15 min, stirring frequently. When the potatoes are fork-tender, add 2 big rainbow chard leaves that have been very thinly sliced. Stir & cover until wilted (about 2 min). Turn the burner off.
Next, beat 6 eggs in a mixing bowl with a bit of salt & pepper then transfer the hot potato mixture to the bowl. Stir. Scrape the skillet to remove any browned pieces that remain then return to a low heat, adding 2 T olive oil to coat the pan. When hot, pour the egg mixture into the skillet. Cover tightly with a lid & let cook on low for 5-8 min. Using a spatula, check to see if the bottom is browning a bit. When it seems mostly intact, using pot holders, place a large plate on the pan & very carefully flip the pan over, transferring the tortilla to the plate, then slide it back into the pan to cook the other side (uncovered) for about 3 min. Cut in wedges, salt to taste & enjoy hot!