Preheat oven to 425°F (220°C).
In large bowl, stir together flour salt and baking powder. In separate bowl, whisk together oil, milk and 1 of the eggs; add to dry ingredients all at once. Using fingers or in mixer using dough hook, blend until liquid is absorbed and dough is smooth. Turn out onto lightly floured surface; knead for about 2 minutes or until velvety smooth. Transfer to bowl; cover and refrigerate for 30 minutes. (Make-ahead: Wrap in plastic and refrigerate for up to 5 days.)
Filling: Brush two large baking sheets with 2 teaspoons (10 mL) of the oil. Cut zucchini, onions and eggplant crosswise into 1/2-inch (1 cm) thick slices; place in single layer on prepared sheets. Brush with 2 tablespoons (25 mL) of the oil; sprinkle with 1/2 teaspoon (2 mL) of the salt. Without peeling, separate garlic into cloves and place in small bowl; add remaining oil and toss to coat. Add garlic to sheets, reserving oil in bowl.
Roast in oven, rotating pans once, for about 40 minutes or until vegetables are tender and garlic is softened. Let cool. (Make-ahead: refrigerate in airtight container for up to 12 hours.) Squeeze garlic from skins in to reserved oil in bowl; mash with fork and set aside.
On lightly floured surface or pastry cloth, roll out pastry to 18-inch (45 cm) circle; transfer to large baking sheet. Spread garlic in 9-inch (23 cm) circle in center of pastry. Top with one-third of the vegetable mixture and one-third of the red peppers; sprinkle with one-third of the cheese, oregano, pepper and remaining salt. Repeat layers twice. Fold pastry border over filling to form attractive irregular edge, leaving 2-inch opening on top. Lightly beat remaining egg; brush over pastry to seal folds.
Bake in lower third of 375°F (190°C) oven for about 30 minutes or until pastry is golden and filling is steaming. Let stand on sheet on rack for about 5 minutes before cutting into wedges.