Preheat the oven's broiler and set the oven rack about 6 inches from the heat source. Grease a baking sheet.
Cut the zucchini in half the long way, and scoop out the seeds and pulp, leaving a thick shell of flesh. Brush both halves of the zucchini with about 1 tablespoon of olive oil, and place them, cut sides down, onto the prepared baking sheet. Bake until the zucchini are hot and beginning to release beads of moisture, 5 to 10 minutes. Remove the zucchini from the oven.
Reduce the oven heat to 450 degrees F (230 degrees C).
Heat 2 tablespoons of olive oil in a skillet over medium-low heat, and cook and stir the garlic and shallot until translucent, about 5 minutes. Remove from the heat and let cool.
Place 1 tablespoon of olive oil, the shrimp, diced tomato, mushrooms, 1/4 cup of Parmesan cheese, basil, and the cooked garlic and shallot into a bowl, and stir to mix. Season to taste with black pepper, salt, and garlic powder. Stuff the mixture into the zucchini halves, and sprinkle each zucchini with about 2 tablespoons of Parmesan cheese.
Bake the stuffed zucchini in the preheated oven until the cheese is browned and the filling is cooked through and hot, about 20 minutes.
