For years, I sliced the cabbage into fine slivers with a Chinese cleaver, but I now use a food processor.
Process the cabbage in batches, watching closely to not make it too fine.
Chop the onion with the cabbage in the food processor, being careful with the quantity because too much onion can overwhelm the flavor of other ingredients.
Combine cabbage/onion mixture with vinegar, mustard, salt, sugar, and celery seeds. Taste to check for desired sweetness.
When the contrast in flavors is correct, add olive oil.
Cabbage flavors vary, as do the flavors of other ingredients, so tasting is the only way to get the desired blend.
Yield: 8 to 10 servings.