Slice the top 1/8 inch off each tomato and carefully remove the core and seeds. Sprinkle the inside of each tomato with a pinch of salt, then place them upside down on several layers of paper towels to drain for 30 minutes.

Adjust an oven rack to the upper-middle position and heat the oven to 375°F. Heat two tablespoons of oil in a large saucepan over medium heat until shimmering. Add the fennel and shallots and cook until softened and lightly browned, 10 to 12 minutes.

Stir in the zucchini and cook until tender, about 5 minutes. Stir in the garlic and cook until fragrant, then add the couscous and cook until the grains are lightly toasted, 1 to 2 minutes.

Stir in the broth and bring to a brief simmer. Remove the pan from the heat, cover, and let it sit for 5 minutes until the liquid is fully absorbed.

Gently fluff the couscous with a fork and stir in half of the Parmesan, the goat cheese, and the basil. Season with salt and pepper to taste.

Pat the inside of each tomato dry with paper towels. Arrange the tomatoes, cut side up, in a 13 by 9-inch baking dish lined with aluminum foil. Brush the cut edges of the tomatoes with the remaining oil.

Mound the couscous filling into the tomatoes, packing it lightly with the back of a spoon. Sprinkle with the remaining Parmesan. Bake until the cheese is lightly browned and the tomatoes are tender, about 15 minutes. Serve warm.