Combine the cabbage, cucumber, tomato, carrot, onion, and pickling salt in a large bowl; toss to combine. Allow to sit 2 hours. Rinse the mixture thoroughly with cold water; drain and squeeze to remove excess liquid.
Combine the vegetables with the water and vinegar in a large pot. Bring the mixture to a boil and allow to boil for 1 hour.
Stir the celery salt, mustard seed, and sugar into the vegetables. Mix the mustard, turmeric, and flour together in a bowl. Carefully spoon 2 tablespoons of the boiling water into the mustard mixture; stir into a paste. Add the paste back into the boiling vegetable mixture; cook and stir until thickened, about 5 minutes more. Cool completely in refrigerator before using.