Melt 2 Tbsp butter in a large pan, add the chopped mushrooms and sauté over medium heat, allowing the mushrooms to soften and the moisture to evaporate. Spread onto a tray and cool.
To make the batter, sift the flour, baking powder, salt and pepper into a bowl, stirring with a whisk to incorporate. Beat the eggs into the milk and add to the flour mixture, stirring to make a thick batter. Add the chopped mushrooms and green onions.
Heat the remaining 2 Tbsps butter in the pan, then spoon the batter into 2 inch, thin circles onto the pan. When bubbles appear on the surface, turn the crepes over and cook for another 20-30 seconds.
Remove to a sheet pan and allow to cool briefly. The crepes may be stacked once cool.
Arrange the crepes on a serving platter or tray. Top with a spoonful of sour cream, then add a few strips of duck prosciutto and, lastly, a little chutney and a sprig of fresh herbs, if desired.