swiss chard recipe

1 bunch Swiss chard, washed and chopped (stems and leaves separated) - 2 tablespoons olive oil - 3 cloves garlic, minced - 1 small onion, thinly sliced - 1/4 teaspoon red pepper flakes (optional) - Salt and black pepper to taste - 1-2 tablespoons lemon juice or balsamic vinegar (optional) - Freshly grated Parmesan cheese (for garnish, optional)