Peel the celeriac and cut in half. Cut the lemon in half. Rub all sides of the celeriac with the lemon.

In a pan with a steamer insert, fill water to below the bottom of the steamer. Squeeze the remaining juice from the lemon into the water. Bring the water to a boil and then reduce to a simmer. Place the celeriac in the steamer insert above the water. Cover and steam until tender, about 30 minutes.

Slice the celeriac halves in half (should be about 1 to 1 1/2-inch wide strips). Cut into 1/4-inch slices and place in a bowl. Pour the vinegar into the bowl and stir gently. Pour the oil into the bowl and stir gently. Season with salt and pepper and stir again. Cover and let set on the counter for at least 1 hour. Stir gently occasionally. Serve at room temperature.