In a small bowl, whisk together the vinegar, mustard, and garlic. Whisking constantly, pour in the olive oil in a slow, steady stream until well blended. Season with salt and pepper and set aside.
Remove the outer leaves from the brussels sprouts and discard any that are blemished. Using a small, sharp knife, remove the cores. Using the slicing attachment on a food processor, pulse the brussels sprouts through until they are all chopped. Remove them from the bowl as necessary until they are done. Alternately, you may chop them by hand.
In a large saucepan over medium heat, melt the butter. Add the chopped brussels sprouts and 1/2 cup water. Cover the pan and raise the heat to high, bringing to a boil. Reduce the heat to medium-low and cook the leaves until bright green and tender, about seven minutes. Add more water if needed. Drain and transfer to a large serving bowl.
Add the bacon to the olive oil dressing mixture. Drizzle it over the brussels sprouts and toss to coat. Season with salt and pepper and serve immediately.