Pierce potatoes with a fork; arrange in a circle on paper towels in microwave oven. Microwave at high 16 minutes or until done, rearranging potatoes after 8 minutes. Let stand 5 minutes.
While potatoes cook, combine beef, flour, salt, and pepper in a large zip-top plastic bag; seal and shake to coat.
Melt butter in a large nonstick skillet over medium-high heat. Add beef mixture and shallots; stir-fry 5 minutes. Add thyme, garlic, and mushrooms; stir-fry 3 minutes. Add broth and wine; cook 5 minutes or until shallots are tender and sauce thickens, stirring frequently. Remove from heat.
Split potatoes open with fork; fluff pulp. Spoon beef mixture evenly over potatoes. Garnish with thyme sprigs, if desired.