Leek and Potato Tart with Gruyère

INGREDIENTS

• 1 (11-ounce) can refrigerated breadstick dough (such as Pillsbury)
• Cooking spray
• 1 teaspoon vegetable oil
• 1 cup thinly sliced leek (about 1 large)
• 1/4 teaspoon black pepper
• 1 medium peeled baking potato, halved lengthwise and cut into 1/4-inch-thick slices (about 1 cup)
• 1 cup fat-free milk
• 1/2 cup egg substitute
• 1/3 cup (1 1/2 ounces) grated Gruyère or Swiss cheese
• 2 tablespoons grated fresh Parmesan cheese
• 1 tablespoon Dijon mustard