In a mixing bowl, combine unsalted butter and white miso paste, then stir well.
Heat a large skillet over medium-high heat. When very hot, add sliced mushrooms and cook until beginning to deeply brown. Use a spatula to stir well, then season with kosher salt and cook. Add the miso butter to the pan, then stir until the mushrooms are evenly coated on all sides.
Heat neutral oil in a large pan over medium heat. When hot, add thinly sliced scallions and shallot, then cook. Add chopped garlic cloves, red chile, ginger paste, and Thai red curry paste. Use a spatula to help break up the curry paste. Season with kosher salt, then cook. Pour in coconut milk.
Add curry leaves, fish sauce, light brown sugar, and fresh spinach. Season with remaining 1 tsp kosher salt. Cook until the spinach has wilted, stirring occasionally, then simmer. Add the juice of lime, stir well, then taste and adjust seasoning as needed.
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!