Sautéed Radishes and Arugula

1 tablespoon unsalted butter
2 tsp. olive oil
1 bunch radishes, greens discarded and radishes halved lengthwise, then sliced crosswise 1/4 inch thick
Salt and pepper to taste
1/2 cup water
1 bunch arugula, coarse stems discarded and cut into 2-inch lengths

Heat butter with oil in an 8-inch heavy skillet over moderately high heat until foam subsides, then sauté radishes with salt and pepper, stirring occasionally, 6 minutes. Add water and cook, covered, until crisp-tender, about 2 minutes, then cook, uncovered, stirring occasionally, until liquid is evaporated, 1 to 4 minutes. Add arugula and sauté, stirring, until wilted, about 1 minute.

Adapted from epicurious.com