Preheat over to 450°F. Place 2 ears of corn directly on the oven rack and roast, turning occasionally, until golden brown, 20 to 25 minutes. Remove from oven and let cool. When cool, cut the roasted corn kernels off the cob. You should have about 1 1/2 cups.
Meanwhile, cook potato in salted boiling water until tender in the center when pierced, 20 to 25 minutes. Drain and let cool to the touch, Slip off the skin and cut the potato into 1/4-inch dice.
With a large, sharp knife, cut the kernels off the remaining ears of corn. Run the kernels through a vegetable juicer. You should have about 4 cups of juice. Combine the corn juice and vanilla bean (or vanilla) in a medium, non-reactive saucepan. Bring to a simmer over medium-low heat, stirring constantly so liquid doesn’t curdle. The natural starch in the juice will thicken it to the consistency of a sauce. If the soup is too thick, thin it with a little water or lemon juice. Remove from heat.
Fish out the vanilla bean (if using) and, with the tip of a small knife, scrape the seeds from the bean into the soup; discard the pod. Blend the soup in a blender at medium speed for a silky-smooth consistency. Return soup to the pot.
Put the roasted corn kernels, beans, and potato in a medium sauté pan or skillet with the water. Bring to a simmer over a medium heat and season with salt and pepper. Simmer for a few minutes until the vegetables are hot. Pour off the water and add the vegetables to the soup. Stir in the shredded spinach or other greens, the lemon zest, lemon juice, salt, and pepper.
If the corn juice curdles during cooking, don’t worry. Beat the curdled liquid with an electric mixer set on medium speed until smooth before you add the rest of the vegetables.