Boudin blanc is partially cooked fresh veal sausage. It must be sauted or grilled before it is eaten.
Heat oven to 400 degrees. Slice sausages in half lengthwise. Place, cut side down, in skillet over medium heat; sear until golden, 5 minutes. Cook 2 to 3 minutes more, until cooked through. Transfer to baking sheet; warm in oven.
To skillet, add oil, garlic, and red pepper. Saute until garlic is fragrant, about 30 seconds. Add broccoli rabe; saute until al dente, about 5 minutes. Season with salt and pepper.
Slice open rolls; toast, about 5 minutes. Spread with mustard. Place one sausage on each roll; mound broccoli rabe over sausage.