Special equipment: a mandoline
Preheat the oven to 425 degrees F.
Roll out the pizza dough to a 10-by-15-inch rectangle on a piece of parchment. Transfer the parchment to a turned-over baking sheet and bake until crisp and golden brown, 10 to 12 minutes.
Meanwhile, use a mandoline to slice the cucumber and radish paper thin. Toss them in a small bowl with 1 teaspoon oil and 1/2 teaspoon salt. Mix together the cream cheese, garlic, herbs and yogurt in a medium bowl.
Transfer the crust to a serving platter or cutting board and spread with the cheese mixture, leaving a 1/4-inch border all around. Arrange the vegetables in a slightly-overlapping single layer. Drizzle with the remaining 2 teaspoons oil and sprinkle with the poppy seeds. Slice into squares and serve at room temperature.
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