Heat a large pot of salted water to boiling.
Heat the olive oil in a very large sauté pan or skillet over high heat. Add the onions and cook until tender. Stir in half the sage. If you don’t mind dirtying another pan, you may brown the potatoes separately. Add the potatoes (browned or not), Swiss chard, green beans, and garlic; cook until the greens are wilted, about 3 minutes.