1. Stir together sour cream and next 5 ingredients (through pepper) in a small bowl with water. Place 1 slice toast on each of 4 serving plates and top each with spinach.
2. In a large saucepan, bring 2 inches of water to a simmer; add vinegar. Working one at a time, crack eggs into a cup and gently slip into water. Simmer 2-3 minutes. Using a slotted spoon, place a poached egg on top of spinach. Spoon about 1 TBSP sauce over each crostino before serving.