In a medium saucepan, combine the 3/4 cup water, oil, lemon juice, sugar and salt. Bring to a boil over high heat. Stir in the carrots. Cover the pan and lower the heat. Simmer the carrots for 3 minutes. Then gently stir in the celeriac so that all the pieces are coated with the sauce. Cover the pan and simmer the vegetables together for about 10-12 minutes longer, or until they are just tender. If a thicker sauce is desired, combine the potato starch and cold water until well combined and stir the paste into the hot sauce. Continue to heat and stir the vegetables until the sauce thickens and simmers. Remove from the heat, serve hot, at room temperature or chilled.