In a large heavy saucepan cook the spinach in the water clinging to the leaves, covered, over moderate heat, stirring once or twice, for 3 to 4 minutes, or until it is wilted, refresh it under cold water, and drain it well in a colander. Squeeze the spinach dry by handfuls and chop it fine. In a bowl stir together well the spinach, the yogurt, the mayonnaise, the cucumber, the radish, the onion, the scallions, the garlic paste, the tarragon, the vinegar, and salt and pepper to taste. Serve the dip with the pita toasts or the crudites. Makes about 2 cups. app104