Heat olive oil in a skillet over medium heat. Cook and stir onion in hot oil until it begins to turn translucent, 3 to 5 minutes. Stir garlic into the onion; cook together about 1 minute. Fold the beet greens into the onion mixture, stirring to coat; cook until the greens wilt, 3 to 5 minutes. Add capers and sun-dried tomatoes; cook and stir until hot, about 1 minute. Remove skillet from heat; season dish with sea salt and black pepper.