Preheat oven to 450°.
To prepare stew, combine mushrooms and 1 tablespoon oil in a single layer on a jelly roll pan. Bake mushrooms at 450° for 30 minutes, stirring once.
Heat 1 1/2 teaspoons oil in a Dutch oven over medium heat. Add the leek, fennel, and carrot; cook 5 minutes. Sprinkle leek mixture with 1/4 teaspoon salt. Cover, reduce heat, and cook 10 minutes. Uncover; add the mushroom mixture, stock, and next 7 ingredients (stock through tomatoes). Bring to a boil. Reduce heat; simmer 5 minutes. Stir in 1/2 teaspoon salt. Combine cornstarch and water. Stir cornstarch mixture into the mushroom mixture, and cook 1 minute.
To prepare gremolata, combine parsley, lemon, and garlic. Serve with stew.