Step 1

Peel and halve the onion lengthwise. Thinly slice the halves and set aside.

Step 2

Bring the vinegar, water, sugar, and salt to a boil in a medium saucepan over medium-high heat, stirring to dissolve the salt and sugar. Remove from the heat, add the onion, and stir to combine, making sure all the onions are submerged in the liquid. Let sit for 15 minutes before using. Store in an airtight container in the refrigerator for up to 1 month.

Step 3

Spread a teaspoon of mustard on each crispbread. Divide the sardines among the crispbreads and gently smash into an even layer with the back of a fork. Place a thin layer of pickled red onions over the sardines (save the remaining onions for another use) and serve immediately.