In a large pot, heat olive oil over medium heat. Add the onion, a few pinches of salt and pepper, and cook for several minutes until the onions are translucent, about 8 minutes.
Add the white wine, stir and let the wine cook off, about 1 minute.
Add the broth, parmesan rind, and white beans. Reduce the heat and let the soup simmer for 5 more minutes. Add the lemon juice and taste and adjust seasonings. Discard the parmesan rind and garnish with fennel fronds.