Wash greens and remove tough centre ribs. Cut leaves into 1-inch strips. (Or purchase pre-washed, stemmed and trimmed fresh greens).
Heat large sauce pot over medium-high heat; add oil and bacon and cook until beginning to brown, about 3 minutes.
Stir in onions and salt; cook until tender, about 5 minutes. Add garlic and pepper flakes, and cook 1 minute more.
Add broth, stirring to scrape up any browned bits on bottom of pan.
Add greens to pot. If necessary, add in batches, stirring each batch to wilt slightly before adding more. Cover and cook until tender, about 25 to 35 minutes.
Stir in vinegar and sugar, and then lower heat to medium. Cover and cook 5 minutes more. Serve hot.
Note: 1 lb of greens equals 2 cups - you can do a mix of greens like above or you can choose 1, whatever you like!