Melt butter in large saucepan. Sweat onion, garlic, bay leaf and thyme until softened but not starting to color. Sprinkle flour evenly, stir well. Pour in liquid while stirring to thicken. Simmer covered 20 minutes. Remove from heat, pull out herbs. Blend with immersion blender until smooth.
In separate small bowl, whisk egg yolks with vinegar. Slowly drizzle into pan while whisking, still off heat. Taste and season with salt and/or pepper as desired – it may seem bland until the right amount of salt balances the other rich ingredients.
Top with rustic croutons or pour over a thick slice of toast. Serves 2 for lunch or first course.
