Begin by crisping the prosciutto. In an oven-safe large pot, heat the olive oil over medium high heat. There should be enough oil to coat the bottom of the pan. Lay the prosciutto in the hot oil and cook, turning once or twice, until browned and crispy, about 4-5 minutes total. Remove to a paper towel lined plate and set aside.
In the same pot, add the onions to the hot oil. Cook for 5-8 minutes until softened, then add the salt, oregano and garlic. Cook for another two or three minutes, being careful not to let the garlic burn. Pour in the wine and let it reduce over medium high heat for another 3-4 minutes.
Add in the tomatoes and cook, stirring occasionally, for 15-20 minutes. If the pot is bubbling and spattering you can place a lid slightly ajar over top. Stir in the spinach, beans, and basil and cover with the lid. Reduce heat to low and let the spinach wilt for 5-8 minutes.
Taste for seasoning, and add up to another teaspoon of salt if needed. Lay the mozzarella slices over the top and place in the oven. Bake for 10-15 minutes until the cheese is bubbling.
To serve, ladle out the beans with the cheese into individual bowls. Crumble the prosciutto over top and enjoy!