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The night before, place the beans in a large bowl and add water to cover by 2 inches. Soak overnight in the refrigerator. The following day, drain the beans, rinse under cold water, and drain again.
Heat the 2 tablespoons of olive oil in a large, heavy-bottomed pot over medium heat. Add the red onion and saute over medium-low heat for 7 to 9 minutes, until wilted. Add the beans and enough water to cover them by 3 inches. Bring to a boil and skim off any foam that rises to the surface. Reduce the heat and simmer gently for 45 minutes, until tender. Drain the beans and onions and transfer to a mixing bowl.
Meanwhile, heat the 1/2 cup of olive oil in a 10- to 12-inch sauté pan over medium-high heat. Add the sun-dried tomatoes, garlic, rosemary, red pepper flakes, and prosciutto and cook for 4 to 5 minutes, until fragrant. Pour the hot oil mixture over the warm drained beans, tossing well. Allow to cool for 10 minutes only. While still warm, stir in the lemon zest, lemon juice, 2 teaspoons salt, and 1 teaspoon black pepper. Place in a large, shallow serving bowl, surround with arugula, taste for seasonings, and serve warm or at room temperature.