Heat the olive oil in a large skillet over medium heat. Add the red onion and sauté until translucent, about 5 minutes. Add the chopped chard, thyme, and 1/4 teaspoon pepper to the skillet. Stir and cook until the chard has wilted, about 2 minutes. Put the chard mixture in a colander in the sink and let cool for 5 minutes, pressing to drain any excess moisture from the greens.