Trim stems from collard greens; chop leaves (about 24 cups lightly packed greens). In extra-large Dutch oven cook greens in lightly salted boiling water for 20 minutes (If using Swiss chard, cook 5 minutes). Drain. Place greens in extra-large bowl of ice water. Allow to stand a few minutes to cool, stirring occasionally. Drain in colander, pressing out excess water. Arrange on tray or baking sheet lined with paper towels; pat dry and set aside.
In a large skillet heat butter over medium heat. Add onion and garlic. Cook and stir about 5 minutes or until tender. Add cream, salt, and pepper. Bring to boiling; reduce heat. Simmer, uncovered, 2 minutes or until slightly thickened. Add greens. Stir to combine; heat through. Makes 8 (1/2-cup) servings.