Brown the Italian sausage in a large pot over medium heat, breaking it up with a spoon as it cooks. Once the sausage is no longer pink, add the diced onion and cook until softened.
Stir in the minced garlic and cook for one minute. Pour in the crushed tomatoes, cannellini beans, chicken broth, oregano, salt, and black pepper. Bring the mixture to a simmer and let it cook for 15 minutes.
While the stew simmers, mix the flour, baking powder, and cheddar cheese in a bowl. Add the milk and melted butter, then stir until a soft dough forms. Do not overmix.
Drop spoonfuls of the dumpling dough onto the surface of the simmering stew. Cover the pot and cook for 15 minutes without lifting the lid. The dumplings should be cooked through and the stew thickened.