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.