Cook mushrooms in batches
Heat a large sauté pan over medium heat, add 3 tablespoons (26g) of the olive oil and heat through. Working in batches if needed to not overcrowd the pan, add the mushrooms and sauté, about 3 minutes.
Add butter and thyme
Add 2 tablespoons (28g) of the butter, thyme sprigs and season with salt and pepper. Continue to sauté until the mushrooms are golden brown, about an additional 3 minutes.
Add shallots and miso
Add the shallots and cook, stirring occasionally, until the shallots are tender, about 3 minutes. Stir in the miso paste.
Add the wine and let reduce by about half, about 4 minutes. Remove the pan from heat then stir in the crème fraîche. Remove the mushrooms to a medium bowl and set aside.
Clean the cast iron pan and return it to heat. Add remaining oil and heat then add the remaining butter and allow to melt. Arrange the pieces of bread to the pan and fry until golden brown and toasted, about 2 minutes.
Assemble and serve
Top the toasted slices of bread with the mushroom mixture and spoon any extra sauce over the top. Garnish with the herbs and pickled shallots (optional).