Meanwhile, in 12-inch nonstick skillet, melt butter over medium-high heat. Add onion, carrot, celery and remaining 1/2 teaspoon salt. Cook 5 to 7 minutes, stirring frequently, until vegetables begin to brown. Add mushrooms; cook 3 to 5 minutes or until mushrooms brown and soften. Add thyme and flour. Cook 1 to 2 minutes, stirring constantly, until flour begins to brown and smells nutty. Add water; simmer 5 minutes, stirring frequently.