In a medium saucepan, combine white wine vinegar with 1 cup (235ml) water, sugar, 1 teaspoon (4g) kosher salt, and tarragon sprigs. Bring to a simmer over medium-high heat, stirring to dissolve sugar and salt.
Place apples and shallots in a stainless steel mixing bowl nested in a larger mixing bowl full of ice water. Pour hot vinegar brine over apples, stirring to chill rapidly. Once cool, discard tarragon and press a clean paper towel against the surface to fully submerge apples. Set aside or place in refrigerator for at least 1 hour.