In a large deep skillet, over a medium heat, add the bacon. Render the fat and cook until crisp and golden brown. At this point, add the onion and cook until translucent. Remove from heat and set aside.
Fill a small saucepan with salted water and bring to a boil over medium heat. Add the sprout leaves and blanch for about 3 minutes. Transfer the leaves to a large bowl of iced water. Drain. Reheat the bacon and onion in the skillet until sizzling, then add the blanched sprout leaves and cook for another 2 minutes.
Toss together well and finish with the butter and fresh tarragon. Season with salt and pepper, to taste and transfer to a serving bowl. Serve immediately.
*For easier dicing and mincing, freeze the bacon until frozen; then dice.