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1
Place cannellini beans in a large container and cover with several inches of cool water; let stand 8 hours to overnight. Drain and rinse.
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2
Melt butter in a heavy pot over medium heat. Cook and stir onion in hot butter until translucent, about 5 minutes. Add garlic; cook and stir until fragrant, about 1 minute.
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3
Stir pork shank, water, chicken stock, and bay leaves into onion mixture; bring to simmer, reduce heat to low, cover the pot with a lid, and simmer until flavors blend, about 45 minutes.
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4
Turn shank over and stir oregano, rosemary, and red pepper flakes into chicken stock mixture; add beans and stir. Return mixture to a simmer, cover the pot with a lid, and simmer until beans are almost tender, about 1 hour. Simmer uncovered until beans are tender, about 30 minutes more. Cool to room temperature, cover the pot, and refrigerate, 8 hours to overnight.
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5
Spoon accumulated fat from the top of the chilled bean mixture and discard fat. Remove pork shank; cut meat into slices and discard bone.
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6
Stir sliced pork shank, tomato, gremolata, salt, and black pepper into bean mixture over medium heat; cook and stir until heated through, 10 to 15 minutes.
