Preheat the oven to 300°. In a large, deep skillet, bring the verjus to a boil. Add the clams, cover and cook over high heat, shaking the pan occasionally. As the clams open, after about 4 minutes, transfer them to a deep, ovenproof platter. Discard any clams that do not open. Cover the platter with a damp kitchen towel and keep the clams warm in the oven. Reserve the clam cooking liquid in a bowl and wipe out the skillet.