Slice the top 1/8 inch off each tomato and carefully remove the core and seeds. Sprinkle the inside of each tomato with salt and place them upside down on several layers of paper towels to drain for 30 minutes—this ensures the tomatoes won't be watery after baking.

Heat the oven to 375 degrees with the rack in the upper-middle position. Heat two tablespoons of the 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.

Add the zucchini and cook until tender, about 5 minutes. Stir in the garlic and cook until fragrant.

Stir in the couscous and cook until lightly toasted and smelling nutty, 1 to 2 minutes. Pour in the broth and bring to a brief simmer, then immediately remove the pan from the heat, cover, and let sit for 5 minutes.

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

Pat the inside of each tomato dry with paper towels and arrange them cut-side up in a foil-lined 13 by 9-inch baking dish. Brush the cut edges of the tomatoes with the remaining oil, mound the couscous filling into the tomatoes, and sprinkle with the remaining Parmesan.

Bake until the cheese is lightly browned and the tomatoes are tender, about 15 minutes. They've done their job when the cheese is golden and the tomatoes are just beginning to slump.