1. Put potatoes in a large, deep pot and cover them with cold water. Add a large pinch of salt and bring to a boil. Cook until soft but not falling apart, 15 to 30 minutes. Remove with a slotted spoon and drain; meanwhile, add greens to water and cook for about 1 minute. Rinse under cold water. Drain well, then chop.
2. Heat oven to 400 degrees. Rice potatoes, run them through a food mill, or mash them with a fork or potato masher, adding enough olive oil to moisten them well. Mash in the greens, adding more olive oil as needed. Sprinkle with salt and lots of pepper.
3. Put mixture in an ovenproof dish and top with bread crumbs. Drizzle with more olive oil, sprinkle with salt and pepper, and bake until bread crumbs are golden brown, about 15 minutes. Serve hot or warm.