Step 1Place sugar, salt, pepper, celery salt, mustard seeds, cayenne pepper, vinegar and water in saucepan and bring to a boil. Reduce heat and simmer until flavors meld, 8 to 10 minutes. Refrigerate 2 to 3 hours before using.
Step 2Remove stem core of tomatoes, then peel and chop into small pieces. Cut stem top and root bottom off onion, peel and cut in half lengthwise. Lay each half flat side down, then cut 1/8 strips lengthwise through each half. Cut into 1/8 inch pieces crosswise and set aside. Peel cucumber and cut into thin slices, then stack slices and cut them in half.
Step 3Add tomato and onion pieces to chilled marinade. Cover relish but stir it gently several times. Keep cucumber pieces separate but refrigerated. At serving time, add cucumber.