Cut the kernels from the cobs. Be sure to scrape the cobs into a bowl to get all the milky pulp. Reserve both. Cut 3 cobs into 1-inch pieces. In a 3-quart saucepan, simmer the cut cobs with the Spanish onion, carrot, and celery in the water for 30 minutes. Strain and reserve the stock.
Simmer the remaining 3 cobs in a 3-quart saucepan with the cream and baking potato over medium heat, until the potato is soft. Be careful not to let the cream boil. Remove the cobs from the cream. Scrape the cobs with the back of a knife and add the scrapings to the cream.
Purée the cream mixture in a blender or food processor, strain, and return it to the pot to simmer.
Meanwhile, heat the butter in a large sauté pan over mediumhigh heat. Cook the red onion, peppers, and reserved corn kernels until tender.
Add them to the simmering cream with the prosciutto and red potatoes. Simmer until the potatoes are tender but not mushy, 10 to 15 minutes. Add the corn stock to thin it to your desired consistency. Season to taste with salt and pepper.
Divide the crab evenly and place in the center of each soup bowl. Top it with a sprinkling of the green onions. Ladle soup over the crab and serve immediately.Servings: 6 to 8