Preheat the oven to 425°.
Arrange the tomatoes cut- side up in a baking dish just large enough to hold them. Mix 1⁄2 teaspoon of the salt and the pepper in a cup; sprinkle over the tomatoes.
Melt the butter in a medium skillet over medium heat. Brush a little butter over the cut side of each tomato, leaving some in the skillet. Bake the tomatoes, uncovered, until they are hot, begin to soften, and look juicy on top, 20–25 minutes.
Meanwhile, add the scallions and garlic to the butter remaining in the skillet. Cook over medium heat, stirring often, until the scallions are tender, about 2 minutes. Add the bread crumbs and cook, stirring, until lightly golden and crisp, 3–5 minutes.
Scrape into a medium bowl. Mix in the Parmesan, pecans, and remaining 1⁄4 teaspoon salt.
Spoon some of the crumb mixture atop each tomato half. Bake until the crumbs are browned and heated, 10–12 more minutes. Serve hot.