Prepare the oven (optional): If you plan to bake the beans in the oven, preheat it to 325°F.
Cook the bacon: Place a large Dutch oven over medium-high heat and cook the bacon until it becomes crisp, about 5 minutes. Remove the bacon and place it on a plate lined with paper towels to drain. Once cooled, crumble the bacon into small pieces.
Sauté the vegetables: Add a bit of olive oil to the Dutch oven if it seems dry. Add the onion and cook until it turns a deep golden brown and caramelized, about 7 to 10 minutes. Stir in the diced parsnip (or carrots), celery root, 2 teaspoons of sage, 2 teaspoons of rosemary, and the garlic. Cook for 1 to 2 minutes until the mixture becomes aromatic.
Simmer the beans: Add the soaked and rinsed beans to the pot, along with the bay leaf, 6 cups of water, and salt. Bring the mixture to a boil, then cover the pot with a lid. You can either transfer the pot to the preheated oven or lower the heat on the stovetop to medium-low. Allow the beans to cook until tender, about 1 ½ to 3 hours. Keep an eye on the liquid level during cooking, adding more water if the beans start to dry out.
Add the finishing touches: Stir in the baby greens, crumbled bacon, the remaining 1 teaspoon of sage, and 1 teaspoon of rosemary. Taste the dish and adjust the seasoning with more salt if necessary. Serve with hot sauce or vinegar for a flavorful finish.