Separate the stems from the beet greens. Finely chop the stems and coarsely chop the leaves.

Heat the olive 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 several grinds of pepper. Sauté, tossing, until just wilted.

Turn off the heat, add the raisins, and squeeze with lemon juice. Toss to combine. Transfer to a platter, top with the walnuts, and season to taste with more salt and pepper.