Place the drained beans in a large pot with enough water to cover by 3 inches/8 cm and bring to a boil. Reduce the heat to medium-low and simmer the beans until al dente, about 1 hour.

Meanwhile, place the spinach, herbs, and leek in a colander and sprinkle lightly with salt. Rub and press the mixture between the palm of one hand and the holes of the colander, almost in a kneading motion, so that the greens exude liquid. Do this for 10 minutes, then place a weighted plate over the greens and leave them to drain for 1 hour.

Heat 2 tablespoons of the olive oil in a medium skillet over medium heat and cook the onions, stirring, until wilted, about 5 minutes. Preheat the oven to 350˚F/180˚C.

Combine the drained greens with the cooked onions and 1 cup of the feta. Drain the beans, reserving 4 cups of the hot liquid. Toss the beans with the greens and 1 cup of the tomatoes. Taste and adjust the seasoning with salt, if desired, and pepper. Place the beans-and-greens mixture in a large ovenproof glass or ceramic baking dish and mix in a ¼ cup of the remaining olive oil and 2 cups of the hot cooking liquid. Pour the remaining 1 cup tomatoes over the surface of the beans and sprinkle the remaining ½ cup feta and the bread crumbs on top. Drizzle ¼ cup olive oil over the surface and bake, covered with a sheet of aluminum foil, until the beans are very creamy and soft but not disintegrating. Depending on the age and condition of the dried beans, this could mean anywhere between 1 and 2 ½ hours of cooking. Add more liquid to the pan if necessary during lukewarm, or at room temperature.