1. Melt butter and olive oil in a medium stockpot over medium-high heat. Add onions and cook, stirring, until onions are brown and caramelized, about 8 minutes. Add carrots, parsnips, and celery; cook until tender, about 15 minutes.

2. Wash and drain Swiss chard thoroughly. Chop into 1-inch pieces.

3. To cooked vegetable mixture, add Swiss chard, 14 cups cold water, thyme, parsley, and bay leaf. Bring to a boil, reduce heat, and let simmer about 1 hour; liquid should be highly flavored.

4. Remove from heat and strain stock through a sieve, pressing on vegetables to extract juices. Discard vegetables. Stock can be refrigerated for 3 to 4 days or frozen for up to 3 months.