Place vinegar in a large heavy-bottomed pot. Stir in sugar, salt, turmeric, garlic, pickling spice and mustard seeds, if using.
Add cucumber and onion. Cover; bring to a boil. Simmer, uncovered, for 10 minutes, stirring occasionally.
Allow pickles to cool slightly. Remove pickling spice and discard. Place pickles into clean glass containers. Spoon liquid over the top. Allow to cool completely.
When cool, cover tightly and refrigerate overnight before serving. May be refrigerated for up to 2 weeks. Makes 4 cups.