Remove any tough or bruised outer leaves from the fennel bulbs. Trim off the root ends; discard. Pull off the feathery tops from the stems. Finely chop enough of the tops to measure 2 tablespoons; set aside. Trim off the stems; discard. Thoroughly rinse the trimmed bulbs and quarter each bulb lengthwise; remove the core. Cut the bulbs crosswise into thin slices. Set aside.
In a very large skillet, cook pancetta or bacon over medium heat 5 to 8 minutes or till browned and crisp, turning once. Using a slotted spoon, remove pancetta, reserving 2 tablespoons drippings in skillet (add salad oil, if necessary). Drain pancetta on paper towels; crumble and set aside.
Add fennel, leeks, and garlic to the reserved drippings in the skillet. Cook and stir over medium heat till the fennel is tender. Remove skillet from heat. Slowly stir dry sherry or vinegar, honey, dry mustard, and pepper into drippings. Return skillet to heat. Bring to boiling. Add the cabbage. Toss mixture in skillet for 2 to 3 minutes till heated through but cabbage is still crunchy.
To serve, transfer cabbage mixture to 6 salad plates or bowls. Top with pancetta, feta cheese, and feathery fennel tops. Serve immediately. Makes 6 side-dish servings.