Gently clean the portobello mushrooms using a damp cloth to remove any dirt. Carefully remove the stems and place the mushrooms, gill side up, on a parchment-lined baking sheet.
In a large mixing bowl, combine the halved cherry tomatoes, mozzarella balls, and chopped basil. Drizzle in the olive oil, sprinkle in the garlic powder, and season with salt and pepper. Mix gently until all the ingredients are evenly coated and combined.
Generously spoon the tomato and mozzarella mixture into each portobello cap, pressing down lightly to ensure they are filled well.
Carefully drizzle the balsamic glaze over each stuffed mushroom, ensuring an even coating for added flavor.
Place the baking sheet in the preheated oven and bake for 20-25 minutes. The mushrooms should be tender, and the cheese should be slightly melted and bubbly.
While the mushrooms are baking, heat a dry skillet over medium heat. Add the pine nuts and toast them for 3-4 minutes, stirring continuously until they turn golden brown. Be vigilant to prevent burning.
Once the mushrooms are done baking, remove them from the oven. Sprinkle the toasted pine nuts over the top and garnish with the reserved whole basil leaves. Serve warm for a delightful appetizer or a light main dish.
Notes
Serve warm for the best flavor. Can be made ahead and baked just before serving.