Add approx. one-third of the butter to a frying pan and fry portobello mushrooms over medium heat. Once they’re starting to brown, add shiitake mushrooms, leek, and garlic. Fry until the leek is translucent, approx. 8 min. Add balsamic vinegar, let reduce, and season with salt and pepper. Remove from the heat, add lemon juice, and set aside to cool.