Trim stems off greens and discard. Slice leaves crosswise into 1/2-inch thick strips or coarsely tear leaves (about 16 cups). In a 6- to 8-quart Dutch oven cook and stir greens in hot oil over medium heat about 5 minutes or until wilted.
Add broth, onions, garlic, and cayenne pepper. Heat to boiling; reduce heat. Simmer, uncovered, for 10 to 15 minutes or just until greens are tender, tossing frequently. Stir in basil, thyme, and, if desired, liquid smoke. Serve immediately.