Wash your Swiss chard throughly (the curly leaves love to capture grit!) and pat dry. Cut the very bottom of the stems off and discard. Slice the rest of the thick stems into 1/2 inch slices. Roughly chop the rest of the leaves (I do this by slicing each leaf into 4 long slices, then roughly chopping crosswise into ~2-inch squares).
Add the olive oil and sliced garlic to a large skillet and cook over low heat, stirring often, until the garlic is tender and just golden, but not browned, about 5 minutes. If the garlic is cooking too quickly, turn down the heat.
When the garlic is tender, add the chard (leaves and stems) and increase the heat to medium. Cook, stirring occasionally, until the chard is just wilted and the stems are crisp tender.
Turn off the heat, squeeze the lemon over the top of the chard, and sprinkle generously with flaky sea salt. Toss quickly to coat, then serve!