Remove stems from collard greens. Wash leaves thoroughly; pat dry, and coarsely chop to equal 20 cups.
Bring 4 quarts water to a boil in a large stockpot; add half of the chopped greens, and cook for 4 minutes. Remove the greens with a slotted spoon, draining them well. Repeat the procedure with remaining chopped greens, and discard the water.
Heat oil in stockpot over medium heat. Add onion and garlic; sauté 3 minutes or until tender. Add greens and salt; sauté 3 minutes or until tender.