Heat a medium skillet over medium heat with oil, 2 turns of the pan. Add onions, garlic, salt, pepper and bay, and cook, stirring frequently, to caramel in color, 30-35 minutes. Add stock and let absorb into onions. Let cool and remove bay leaf.
Combine the onions with the crème fraiche or sour cream, mustard and egg yolks; adjust seasoning to taste. Fill a food storage bag with egg mixture and trim a corner with scissors to form a disposable pastry bag.
Fill eggs and top with crispy shallots or fried onions.