To prepare the sauce, combine the tofu, mayo, and lemon juice in a blender and blend until it's smooth. Transfer it to a bowl with a lid, and stir in horseradish, salt to taste, and pepper. Close the container, and refrigerate until needed.
Next, heat a large skillet over medium heat. Add the sliced onion, 1 tbsp of oil and a large pinch of salt. Cook the onions down for 25-30 minutes, until they’re nice and caramelized, stirring frequently to prevent burning.
Once onions are done, transfer them to a plate and set aside. Add another tbsp or 2 of cooking oil and add the seitan. Saute for 2-3 minutes until it's warm. Add in the French onion dip mix, Worcestershire sauce, and water. Stir until all of the pieces are coated in flavoring. Then add the onion in with the seitan.
Load your sandwiches up with horseradish cream sauce, seitan, onions, and cheese. Microwave the cheese for 10 seconds or heat it up in a pan with a bit of water if you like it melty! Serve warm and enjoy.