Place the beans in a large pot with water to cover.
Bring to a boil over high heat.
Meanwhile, place the dried mushrooms in a separate bowl and cover them with hot water to soak.
Cut a small slit in the side of the unpeeled onion and insert the bay leaf.
Add the onion to the pot with the beans.
Reduce the heat to medium-low, cover partially, and let the beans simmer gently, stirring occasionally.
Once the mushrooms are soft, lift them out of their soaking liquid, reserving the liquid for later.
Squeeze the mushrooms to remove excess water, trim any tough spots, and give them a rough chop.
After 30-60 minutes, when the beans begin to soften, season generously with salt and pepper. Carefully pour the reserved mushroom-soaking liquid into the pot, leaving any grit behind.
Continue to simmer until the beans are fully tender but still intact.
Stir the chopped mushrooms into the pot and cook for another 10 to 15 minutes to meld the flavors.
Remove and discard the onion and bay leaf.
Taste and adjust seasoning if needed. About 3 minutes before serving, stir in the minced garlic, fresh sage, and olive oil (or melted butter). Spoon into individual bowls and serve immediately.
Don't be afraid to swap the beans! This recipe works beautifully with cannellini, pinto, kidney beans, or even green or brown lentils.