Melt 2 tablespoons butter in a large skillet over medium heat. Stir in 1 pound (2 bunches) trimmed, quartered radishes and 1/4 teaspoon sugar. Add 1/3 cup water, 1/4 teaspoon grated lemon zest and 1/2 teaspoon kosher salt. Partially cover and simmer until almost tender, 5 minutes. Uncover; cook, stirring, until most of the liquid evaporates. Season with lemon juice and chopped tarragon.