For the pickled onions:
1 small red onion (about 5 ounces)
1/2 cup rice vinegar, white wine vinegar, or apple cider vinegar
1/2 cup water
1/2 teaspoon granulated sugar
1/2 teaspoon fine salt
For the toasts:
4 Wasa crispbreads
4 teaspoons Dijon mustard
1 (3.5 ounce) can sardines
For the pickled onions: Peel and halve the onion lengthwise. Thinly slice the halves and set aside.
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.
For the toasts: 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.
Recipe Notes
- Sardines: Oil- or water-packed sardines are both fine to use in this recipe; just use a fork to lift the fish out of the packing liquid.