Make the miso butter. In a mixing bowl, combine 3 Tbsp unsalted butter and 2 Tbsp white miso paste, then stir well.
Cook the mushrooms. Heat a large skillet over medium-high heat. When very hot, add sliced mushrooms and cook undisturbed for 6-8 minutes, or until beginning to deeply brown. Use a spatula to stir well, then season with ½ tsp kosher salt and cook for an additional few minutes on the other side. The mushrooms should be golden brown on both sides. Add the miso butter to the pan, then stir until the mushrooms are evenly coated on all sides. Either transfer the mushrooms to a bowl and wipe out the skillet or grab another skillet.
Make the curry sauce. Heat 2 Tbsp neutral oil in a large pan over medium heat. When hot, add thinly sliced scallions and 1 diced shallot, then cook, stirring occasionally, for 2 minutes. Add 3 chopped garlic cloves, 1 chopped red chile, 2 tsp ginger paste, and 2 Tbsp Thai red curry paste. Use a spatula to help break up the curry paste. Season with ½ tsp kosher salt, then cook down for 2 minutes. Pour in a 13.5oz can of coconut milk.
Add the beans. Add 2 cans of cannellini or butter beans, along with the liquid in the can. Stir well.
Simmer with remaining ingredients. Add 2 curry leaves, 2 tsp fish sauce, 1 Tbsp light brown sugar, and 5 oz fresh spinach. Season with remaining 1 tsp kosher salt. Cook until the spinach has wilted, stirring occasionally, then simmer for about 5 minutes. Add the juice of 1 lime, stir well, then taste and adjust seasoning as needed.
Serve with mushrooms! Divide the white bean curry evenly between bowls, then top each with some of the miso butter mushrooms. Garnish with fresh herbs, such as cilantro, basil, or mint and additional thinly sliced scallions. Serve with lime wedges. Enjoy!