This is a great tasting, rich side dish. Pair it with steak or a similarly robust main where you'd typically eat mashed potato. The secret is to make sure the liquid reduces fully each time you add it to the mushrooms.
Heat olive oil in a large skillet over medium high heat. Add onion and sautée until soft and transparent all through. Add mushrooms, salt well, and sautée until the mushrooms release their liquid. Continue to sautée until most of the mushroom liquid is boiled off. Add 1 cup chicken broth and bring back to a boil. Continue to cook until the broth is almost entirely cooked off. Add wine, bring to a boil, and cool until the wine is almost entirely cooked off. Add the tarragon and the remaining broth and cook once again until it's almost entirely cooked off.
Add the beans, mix thoroughly, and cook until the beans are cooked through. Remove from heat, add butter, and stir until the butter is completely melted and mixed into the sauce.