Rinse the potatoes and put them in a saucepan with cold water to cover and salt. Bring to a boil and simmer for 15 minutes. They will not be fully cooked. Drain the potatoes and when they are cool enough to handle, peel and cut them into half-inch slices.
Pare the celeriac, cut it into quarter-inch slices and put them in a saucepan with water to cover. Add the lemon juice and salt. Bring to a boil and simmer for 10 minutes. Drain and reserve a half cup of the cooking liquid.
Preheat the oven to 375 degrees. Heat the oil in a skillet. Add the garlic, tomatoes, salt, pepper and basil leaves. Cook, stirring, for one minute. Add the cream, the reserved half cup of cooking liquid, nutmeg, salt and pepper. Bring to a boil and simmer for two minutes.
Grease an oval au gratin dish, measuring 2 1/2 by 8 by 10 inches, with butter. Arrange the potatoes in one layer. Sprinkle half of the grated cheese over the potatoes. Place the celeriac slices over that. Pour the tomato mixture over all and then the remaining grated cheese. Place on the bottom rack of the oven and bake 40 minutes until nicely browned.