Separate the stems from the beet greens. Finely chop the stems and coarsely chop the leaves.
Heat the oil in a large skillet over medium heat. Add the garlic and the beet stems and cook, stirring, for 1 minute. Add the beet greens, a few pinches of salt and freshly ground black pepper, and sauté, tossing, until just wilted.
Turn off the heat, add the raisins, a big squeeze of lemon, and toss. Transfer to a platter, top with the walnuts and season to taste with more salt and pepper.