Peel celeriac and cut into quarters. Using a mandolin or sharp knife, thinly slice celeriac.
Add lemon and salt to water, then add celeriac slices, stirring so they don’t clump. Cook until just tender, 7-10 minutes. Remove from pot and drain thoroughly, pressing out water, and allow to cool.
Heat oven to 400 degrees. Butter a two-quart baking dish and, while oven preheats, heat tomato sauce and cream in a pot over low heat.
Layer a third of the celeriac slices on the bottom of the buttered baking dish. Top with a third of the tomato sauce, and a third of the grated cheese. Repeat twice more.
Bake until golden brown and bubbly, about 40 minutes. Cut into wedges and serve.