Heat the oil over a medium heat in a large, deep saute pan or 8-quart stockpot. Saute the onion until soft and translucent, about 5 minutes. Add the garlic and cook an additional 1 minute. Stir in the sausage and cook until heated through and browned, about 4 minutes. Add the escarole and cook until wilted, about 3 to 4 minutes. Add the beans, 1 cup of chicken stock, sage and red pepper flakes and simmer until the mixture is heated through and liquid is slightly reduced. Add the
sausage-bean mixture to pasta and toss well, loosening with the additional 1/2 cup chicken stock if necessary. Season with freshly ground pepper and salt, to taste.
Divide among 4 pasta bowls and top with parmesan cheese.