Preparation Preheat oven to 350°F. Melt butter in an ovenproof 10" skillet (such as cast iron) over medium-low heat. Add leeks and ramp bulbs and cook, stirring often, until leeks are softened, about 10 minutes. (Try to keep the leeks from browning.) Add ramp greens, arugula, and spinach in big handfuls, cooking and wilting one batch before adding the next. (At first it will look like all those greens will never fit in the skillet, but keep at it—you’ll see!) When all greens are wilted (this may take 15–20 minutes), add coriander, salt, and cayenne. Cook, stirring, until spices are incorporated, about 1 minute. Add lemon zest, cream, and 1/2 cup Parmesan. Continue to cook, stirring, until cream has slightly reduced, about 2 minutes longer. Remove from heat. Toss bread with oil in a large bowl to coat. Arrange bread on top of vegetables, taking care not to cover the greens completely (you want them to peek through here and there). Sprinkle remaining 2 Tbsp. Parmesan over. Transfer to oven and bake gratin until bread is golden brown and crisp, cheese is melted, and cream is bubbling, 35–45 minutes. Let cool slightly before serving.