Heat oven to 425 degrees. Place potatoes in a medium stockpot, cover with water, and add 1 tablespoon salt; bring to a boil. Reduce heat to medium high, and boil gently until potatoes are tender, about 45 minutes. Drain potatoes in a colander. When cool enough to handle, peel potatoes, and pass the flesh through a potato ricer into a large metal bowl. Cover with aluminum foil, and place over a pot of simmering water.
Meanwhile, in a medium bowl, combine celeriac, 1 tablespoon olive oil, 1 tablespoon chicken stock, 1/2 teaspoon salt, and 1/4 teaspoon pepper. Transfer to a shallow baking pan, and roast until celeriac is browned and tender, about 35 minutes. Remove from oven, and stir celeriac into potatoes. Cover bowl of potatoes again.
In a small saucepan, combine cream, milk, and 4 tablespoons butter; warm over medium-high heat until butter melts and milk just comes to a boil, about 2 minutes. Stir into the potato and celeriac mixture along with 2 tablespoons salt and 1/4 teaspoon pepper. Cover bowl again.
In a large skillet, heat remaining tablespoon olive oil and remaining 2 tablespoons butter over medium-high heat. Add the sturdiest mushrooms to skillet. Saute, stirring frequently, until mushrooms are lightly browned, 2 to 3 minutes. Add remaining mushrooms, 3 teaspoons rosemary, 1/2 teaspoon salt, and 1/4 teaspoon pepper. Cook until mushrooms are tender, 3 to 5 minutes. Add sherry and remaining 2 tablespoons stock, and cook until mushrooms absorb the liquid, 2 to 3 minutes.
Transfer potatoes and celeriac to a serving dish, and cover with mushrooms. Sprinkle with the remaining 1 1/2 teaspoons rosemary, and serve immediately.