Preheat oven to 350 degrees. Stir together yogurt, 2 tablespoons scallions, the garlic, salt, and 1/8 teaspoon pepper in a bowl. Spoon 1/2 cup yogurt mixture into each of four 6-ounce ovenproof dishes. Place on a rimmed baking sheet. Bake until yogurt is just firm, 10 to 15 minutes.
Meanwhile, fill a large straight-sided skillet with 1 inch water. Bring to a simmer over medium-low heat. Working quickly, crack each egg into a small cup, and gently slide egg into water. Simmer until whites are opaque but yolks are still runny, 2 to 3 minutes. Using a slotted spoon, transfer eggs to a plate lined with paper towels, and let drain.
Place 1 egg on top of each baked yogurt. Top with watercress, tomatoes, and remaining tablespoon scallions. Sprinkle with 1/4 teaspoon pepper and the paprika. Serve immediately with toast.