Preheat oven to 375 degrees Fahrenheit. Grease two baking sheets with nonstick cooking spray.

Clean portobello mushrooms and remove stems. Drizzle with balsamic vinegar. Place on one baking sheet and bake for 10 minutes.

In a medium bowl, combine ground chicken, barbecue sauce, and onion powder. Form a burger with a width that is a bit smaller than the portobello mushrooms. Place on second baking sheet and bake for 10 to 15 minutes.

Meanwhile, add potatoes to a large pot. Cover with water and bring to a boil. Cook for 20 minutes, until tender. Drain and mash.

In a small frying pan, heat olive oil. Add onion and sauté until translucent. Add onion to mashed potatoes and season with salt.

In the same small pan, heat olive oil. Add onion and sauté for five minutes.

Add garlic and diced tomatoes. Bring to a simmer. Let cook for 10 minutes and season with basil, salt, and pepper.

Cook for three to five additional minutes. Remove from heat.

Stack as follows: Portobello mushroom upside down, chicken burger, mashed potatoes, and salsa topping.