Look for radishes with fresh, vibrant green tops, which lend a striking color and mild peppery bite.
Melt butter in a large skillet over medium-high. When it boils, reduce heat to medium; simmer until foamy. Continue cooking, stirring occasionally and scraping bottom of pan, until foam subsides, butter turns golden brown with a nutty aroma, and milk solids separate into brown specks that sink to bottom, 2 to 7 minutes. Remove from heat. Add radishes, kohlrabi, Lillet, and 3/4 cup water. Generously season with salt and pepper. Boil, stirring occasionally, until vegetables are crisp-tender and liquid is reduced to a glaze that evenly coats vegetables, about 15 minutes. Transfer to a serving bowl; let cool 5 minutes. Toss with radish greens; serve.