Bring large pot of salted water to a boil, add the potatoes and cook until just almost tender, about 20 minutes.
Wash chard and trim stems, cut the stems into 1 inch pieces. Fold leaves together, and cut into 1/2 inch strips. Add stems from chard to boiling potatoes and cook another 10 minutes, add the leaves and cook until wilted. Drain potatoes and chard very well in colander.
In large heavy skillet, heat oil over medium heat and add garlic. Season oil with salt and pepper and add red pepper flakes. Add chard and potatoes, cook over medium heat, stirring often, and mashing potatoes gently as they cook, for about 8 minutes. Taste, and adjust seasonings as needed, serve warm.