Step 1

Slice squash into very thin rounds. Remove both onion ends, peel, and cut in half lengthwise. Slice 1/2 of the onion into very thin strips, reserving the other half for another use. Seed and finely dice the bell pepper.

Step 2

Toss squash, onions, peppers, and salt together in a mixing bowl. Add a handful of ice cubes and just enough cold water to cover. Stir well to combine, and set aside for thirty minutes.

Step 3

Rinse the vegetables well and place in a colander or sieve to drain. Meanwhile bring the vinegar, sugar, and ground mustard to a boil in a small saucepan. Immediately remove from heat, stirring to make sure the ingredients are dissolved, and allow the vinegar to cool for 3-5 minutes.

Step 4

Gently pat vegetables dry with a towel and place in an airtight container. Pour vinegar over the mixture, tossing a fork to make sure all the slices get soaked. Cover and refrigerate until cool. Strain from brine before serving.