How to Make It
In a large saucepan, combine 1/2 cup of water with the tomato sauce, ketchup, lemon juice, olive oil, tarragon vinegar, Worcestershire sauce, horseradish, brown sugar and garlic. Add the cayenne pepper and season with salt. Bring to a boil over high heat, then simmer for 20 minutes. Transfer the marinade to a large bowl to cool.
In a large saucepan of boiling salted water, cook the cauliflower until barely tender, about 2 minutes. Using a slotted spoon, transfer the cauliflower to a baking sheet to cool. Bring the water back to a boil. Add the carrots and celery, blanch for 2 minutes and transfer to the baking sheet to cool. Add the white wine vinegar to the saucepan and return to a boil. Add the mushrooms, simmer for 1 minute and drain well; transfer to the other vegetables and let cool.
Add all of the cooled vegetables to the marinade, then add the remaining ingredients. Toss gently but thoroughly, cover and refrigerate overnight.