Rinse sprouts, drain, and discard discolored leaves. Using a small, sharp knife, trim stem end of each sprout to release a layer of leaves, then pull the leaves from the sprout (save). Continue to trim and discard stem, pulling off (and saving) leaves until only about 1/2 inch remains of center of sprout; cut this portion in half lengthwise. Repeat to separate leaves on remaining sprouts.
Cut bacon into 1/4-inch dice. In a 14-inch wok, 14-inch frying pan, or wide 6- to 8-quart pan, stir bacon often over medium-high heat until browned, about 10 minutes; transfer with a slotted spoon to paper towels to drain. Pour bacon drippings from pan and reserve, if desired for dressing.
Meanwhile, in a small bowl, mix 5 tablespoons olive oil, 5 tablespoons vinegar, shallots, garlic, thyme, 1/2 teaspoon salt, and 1/4 teaspoon pepper. Peel onion and cut lengthwise into thin slivers.
Return 1 tablespoon bacon drippings to pan or add 1 tablespoon olive oil; add sliced onion and 1 tablespoon vinegar. Stir often over medium-high heat until onion is limp and lightly browned, about 8 minutes. Pour into bowl with vinegar and olive oil mixture.
Add another tablespoon bacon drippings (discard any remaining) or olive oil to pan over high heat; when hot, add brussels sprout leaves and stir until limp, 3 to 4 minutes. Add croutons, bacon, and oil-and-vinegar mixture to pan; stir until hot and well mixed. Pour into a wide bowl and sprinkle with chopped egg. Add more salt and pepper to taste.