1 Small pot, 1 Citrus juicer, 1 Tablespoon, 1 Pot, 1 Teaspoon, 1 Measuring cups, 1 Metal bowl, 1 Hand mixer, 1 Tablespoon
1 Melt the butter in a small saucepan. Squeeze juice from lemon.
2 Fill a wide pot about halfway with water. Heat the water until it steams, but is not boiling.
3 Separate the eggs (reserve the whites for another use). Mix the yolks with mustard and vegetable stock in a small pot and then set this pot over the pot of steaming water. (The bottom of pot should not touch the water below.)
4 Beat the yolk mixture with a hand mixer or whisk unitl creamy and light.
5 Whisk in yogurt, 1 tablespoon at a time, then whisk in the melted butter in a thin stream until combined.
6 Season sauce with 1-2 teaspoons lemon juice, salt and pepper. Mix or whisk again to make smooth. Place in a preheated gravy boat and serve immediately.