Canned artichoke bottoms make this a quick-and-easy spring appetizer. The artichoke bottoms can be prepared a day ahead and baked just before serving.
Heat 1 Tbs. oil in skillet over medium heat. Add leek, and sauté 5 minutes, or until translucent and soft. Stir in spinach and garlic, and cook 3 minutes, or until most of liquid has evaporated. Remove from heat, and stir in feta until smooth. Season with salt and pepper. Cool 10 minutes, or until easy to handle.
Preheat oven to 350°F. Coat baking sheet with cooking spray, and place artichoke bottoms on baking sheet. Fill each artichoke bottom with spinach mixture (amount will depend on size of artichoke bottoms), and top with tomato slice. Sprinkle each tomato slice with salt, pepper, and oregano, then with breadcrumbs. Drizzle with remaining olive oil. Bake 25 to 30 minutes, or until breadcrumbs are browned and crispy.