Theses pickled red onion petals are slightly saltier than your average pickled onion, but the brine amps up the fresh in the mussels, making them a perfect pair.
Place onion slices in a medium-size heatproof bowl. Bring 1 1/2 cups water, sugar, vinegar, and salt to a boil in a saucepan over high, stirring occasionally to dissolve sugar. Pour vinegar mixture over onions, and place a small plate directly on onions to keep them submerged. Let onion mixture stand at room temperature 2 hours before serving. Drain pickled onions before using.
Pickled red onion petals may be covered and stored in refrigerator up to 1 week.