In a small bowl, combine 1½ cups (360 grams) warm water, 1 teaspoon (4 grams) sugar, and yeast. Let stand until foamy, 5 to 10 minutes.
In the bowl of a stand mixer fitted with the paddle attachment, beat flour, dry milk, kosher salt, xanthan gum, baking powder, and remaining 3 teaspoons (12 grams) sugar at low speed just until combined.
Add the yeast mixture and ¼ cup (57 grams) oil. Beat at medium speed until combined and elastic, about 5 minutes.
Cover and let stand at room temperature for 30 minutes.
Generously spray a 13x9-inch baking pan with cooking spray. Line pan with parchment paper; spray parchment.
Transfer dough to prepared pan. Using wet hands, carefully spread and press dough toward edges of pan in an even layer.
Using your fingers, dimple the surface. Cover and let stand for 30 minutes.
Position oven rack in center of oven. Preheat oven to 400°F (200°C).
Drizzle remaining 2 tablespoons (28 grams) oil over dough. Top with tomatoes, oregano, and pepper.
Place pan in oven, and immediately reduce oven temperature to 375°F (190°C). Bake until top is puffed, about 20 minutes.
Carefully top with leek. Bake until focaccia is golden, about 10 minutes more.
Let cool in pan for 10 minutes. Remove from pan, and place on a wire rack.
Serve warm, or let cool completely.
