Preheat oven to 350°.
Place celeriac and potatoes in a saucepan; cover with water, and bring to a boil. Reduce heat; simmer 12 minutes or until potatoes are tender. Drain.
Bring the milk to a boil in a saucepan over medium heat; add apple, reduce heat, and simmer, uncovered, 7 minutes.
Place potato mixture, apple mixture, 1 1/2 teaspoons butter, and salt in a food processor; process until smooth. Set aside.
Melt 1 tablespoon butter in a large nonstick skillet over medium-high heat. Add onion, carrot, and celery; saute 5 minutes. Add turnip; cook 3 minutes. Add mushrooms, parsley, savory, and thyme; cook 2 minutes or until vegetables are tender and liquid almost evaporates. Sprinkle with flour; stir well, and cook 2 minutes. Stir in apple juice; cook 2 minutes. Cool to room temperature. Stir in cheese.
Steam spinach, covered, 5 minutes or until done. Drain and coarsely chop.
Coat 6 (12-ounce) souffle dishes with cooking spray. Spoon 1/2 cup potato mixture into each dish. Top each with 1/3 cup spinach; top spinach with 2/3 cup onion mixture. Spoon remaining potato mixture evenly into each dish. Sprinkle evenly with breadcrumbs. Bake at 350° for 30 minutes.