Preheat the oven to 400°. Bring a large skillet of salted water to a boil. Cut the green leafy tops off the ramp bulbs. Add the greens to the skillet and boil until tender, about 2 minutes. With a slotted spoon, transfer the greens to a colander. Add the ramp bulbs to the skillet and boil until just tender, about 4 minutes. Drain the bulbs and cut them in half lengthwise. Squeeze the greens dry and coarsely chop them.
Butter an 8-by-10-inch shallow baking dish. In the skillet, melt the butter. Add the tomato and cook over high heat for 1 minute. Add the ramp greens and bulbs along with the cream and season with salt and pepper. Simmer over moderate heat, stirring gently, until the cream is slightly reduced, about 2 minutes. Transfer the contents of the skillet to the prepared baking dish. Sprinkle with the cheese and bake for about 10 minutes, until bubbling.
Preheat the broiler. Broil the gratin until browned on top and serve hot.