Melt the butter in a medium skillet over medium heat. Add the leeks and radishes and cook until slightly softened, about 4 minutes. Add the asparagus and radish tops, if using, along with 1/4 teaspoon salt, a few grinds of pepper and 1/4 cup water. Bring the water to a simmer, then cook, covered, until the asparagus are just barely crisp-tender, about 2 minutes. Uncover, increase the heat to medium-high and cook, stirring, until most of the liquid has evaporated and the vegetables are all crisp-tender, about 2 minutes. Stir in the tarragon, lemon zest and juice. Season with salt and pepper.