Using the flat of a knife, mash garlic with salt to a paste. Whisk together garlic paste, mustard, honey, and vinegar, then add oil in a stream, whisking until emulsified. Let stand at room temperature until Brussels sprouts are cooked.
Slice Brussels sprouts in half, remove cores and gently pull the leaves apart. Heat large sauté pan and add olive oil and pancetta. Render fat from pancetta until lightly crispy. Remove pancetta and reserve.
Add onions and cook until translucent, about 5 minutes. Add butter and swirl to melt. Add the Brussels sprouts and 2 tablespoons of water. Sautee over medium heat, cooking just until leaves are tender and bright green, about 6-8 minutes.
Fold pancetta into warm greens and turn into serving bowl. Toss with dressing to taste and serve immediately.