Pierce potatoes with a fork; arrange on paper towels in microwave oven. Microwave at high 10 minutes or until done, rearranging potatoes after 5 minutes. Let stand 5 minutes.
Cut each potato in half lengthwise; scoop out pulp, leaving a 1/4-inch-thick shell. Combine potato pulp, 1/4 cup cheese, sour cream, salt, and pepper. Spoon potato mixture evenly into shells.
Melt butter in a medium nonstick skillet over medium-high heat. Add onion and sugar; sauté 8 minutes or until browned. Stir in sherry, Worcestershire, thyme, and garlic; cook 1 minute or until liquid evaporates, scraping pan to loosen browned bits. Top each potato half with about 2 tablespoons onion mixture and 1 tablespoon of cheese. Arrange stuffed potato halves on paper towels in microwave oven. Microwave at high 1 minute or until thoroughly heated.