Heat 1 tablespoon olive oil in a large (12-inch) saute pan over medium heat. Add the bacon and cook, stirring occasionally, for 5 to 7 minutes, until browned and crisp. With a slotted spoon, transfer the bacon to a plate lined with a paper towel and set aside.
Add the onion and shallots to the pan and saute for 5 minutes, until lightly browned.
Add the Brussels sprouts (including any loose leaves), 2 teaspoons salt, and 1 teaspoon pepper, toss well, and cover. Cook for 5 minutes
Add the butter and cook uncovered for 5 more minutes, until the sprouts are tender.
Stir in the cooked bacon and balsamic vinegar and stir to deglaze the pan. Sprinkle lightly with fleur de sel and serve hot.