Mixed Berry Shrub Cocktail

Name: Mixed Berry Shrub Cocktail
Ingredients: For the mixed berry shrub:; 2 large Driscoll's strawberries, trimmed and finely sliced; 1 1/2 pints (8 ounces) mixed Driscoll's berries, such as blueberries, raspberries, and/or blackberries; 1 cup (8 ounces) apple cider vinegar; 1 cup (8 ounces) sugar; 1 teaspoon kosher salt; For the cocktail:; 1 1/2 ounces (2 tablespoons) mixed berry shrub; 1 1/2 ounces (2 tablespoons) red bitter amaro, such as Campari or St. Agrestis Bitter; 3-4 ounces (1/2 cup) chilled seltzer or prosecco; Frozen finely sliced strawberries, for garnish