Rinse the chard well. Cut the stems away from the leaves, chop the stems into 1 cm pieces, and roughly chop the leaves.
Bring a 6-quart / 5.7 L pot of well-salted water to a boil. Add the chopped stems and blanch for 2 minutes.
Add the leaves to the pot and blanch both together for another 2 minutes, until the leaves are wilted and the stems are just tender.
Drain the chard in a colander, rinse briefly under cold water, then squeeze it firmly with your hands to remove as much water as possible.
Heat the olive oil in a 12-inch / 30 cm skillet over medium heat. Add the sliced garlic and chili flakes and cook for 45 to 60 seconds, until the garlic turns golden at the edges and smells fragrant. Watch it closely, as it burns fast once it starts to color.
Add the drained chard to the skillet and toss to coat in the garlic oil. Raise the heat to medium-high and cook for 4 to 5 minutes, tossing occasionally, until the chard looks glossy and any leftover water has evaporated.
Season with salt to taste and add more chili flakes if you want extra heat.
Finish with a squeeze of lemon if using, transfer to a serving dish, and serve warm.