Preheat the oven to 375 degrees F and butter a shallow 3-quart baking dish. Melt 6 tablespoons butter in a large nonstick skillet over medium heat. Add the scallion whites, bell pepper, thyme and celery; cook, stirring occasionally, until the vegetables are softened, about 6 minutes. Add the broth, 1 and teaspoon salt, bring to a simmer, then remove from the heat.