Preheat oven to 350 degrees Fahrenheit (175 degrees Celsius).

Thinly slice potatoes, sweet potatoes, and onions in a food processor. Mix together in a large bowl. Drizzle some olive oil on the bottom of a round casserole or Pyrex dish. Pour potato mixture in.

In a medium-sized pot, heat 1/4 cup olive oil. Add flour and stir until blended. Whisk in soy milk and chicken soup mix dissolved in one cup boiling water. Cook, stirring constantly, until mixture boils and thickens. Add salt and pepper.

Blend or puree cooked celery root in a food processor with remaining 1/4 cup water until smooth. Add this to the sauce and stir to combine. Remove from heat. Pour sauce over potato mixture in the pan. Sprinkle paprika on top. Line a baking sheet with parchment paper to catch drippings that might bubble over. Place dish on the sheet and bake for 45 minutes, or until potatoes are tender and golden brown.

Heat oil in a small pot over low heat. Add flour and mustard, and stir to form a paste. Cook for two to three minutes. Whisk in soy milk.

Bring to a boil and simmer for five minutes, whisking often to ensure a smooth, lump-free sauce. Remove from heat.

Add fresh parsley and lemon juice, if using. Add salt and pepper. Mix together well. Cover with plastic wrap to prevent a skin from forming. Serve warm, spooning over the scalloped potatoes immediately before serving. (If the sauce thickens in the fridge, dilute with a little soy milk or water before rewarming.)