Heat the olive oil in a 12-inch cast iron or heavy skillet over medium-high heat. Add the bacon and cook until browned and crispy, then turn and cook the other side, about 3 minutes total. Remove the bacon from the pan, and transfer to a paper towel lined plate.
Cook the onion in the hot fat in the pan until it softens, about 3 minutes. Add the finely diced potato and chard stems, reduce heat to medium and sauté, stirring now and then until tender, about 4 minutes.
Add the Swiss chard leaves in bunches, adding more greens as they wilt and room allows. Add the pinch of salt, nutmeg, pepper, and garlic and cook for about 3 minutes or until the greens are tender.
Beat the eggs with the salt, pepper, and pinch of cayenne. Set the oven rack to the second highest position and preheat the broiler.
Crumble and sprinkle the reserved bacon and cheese into the skillet and pour the eggs evenly over them. Reduce heat to medium low and cook for 2 minutes. Transfer the pan to the broiler and broil for about 4 minutes or until the eggs are set and the top has browned.
Remove the frittata from the oven and let rest for 3 minutes before cutting into wedges. Serve hot or at room temperature.