In a 9-inch ovenproof nonstick skillet, bring 1/4 cup of water to a boil over moderately high heat. Add the spinach and cook, stirring, until wilted, about 1 minute. Using tongs, transfer the spinach to a colander. Repeat with the arugula. Gently squeeze the greens dry and coarsely chop them. Wipe out the skillet.