1. Preheat oven to 400°F. Cut bread on a slight angle into 1/4-inch-thick slices. Arrange in a single layer on a baking sheet. Brush one side of each slice with a small amount of oil. Sprinkle with salt and pepper. Toast until lightly browned, 15 to 20 minutes, turning pan around twice during baking time. Allow to cool completely.
2. In a small bowl, combine avocado, mango, cilantro, onion and lime juice. Season with salt and pepper and mix gently to combine.
3. Mound 1 Tbsp. mango mixture on top of each toast just before serving.