Soak the shiitake in warm water for 15 minutes, then drain the liquid and rinse the mushrooms. Wash the bell pepper, remove the stem, internal membranes and seeds. Peel the onions. Cut the shiitake and onion into small pieces, and the bell pepper into strips. Grind the Parmesan using a medium-mesh grater. Prepare a heatproof baking sheet. For serving, wash, dry and chop the dill. Turn the oven on 180 degrees.
Prepare onions with shiitake. Saute the onions in a frying pan with oil for 3-4 minutes. Add the shiitake and fry until tender, stirring occasionally, for 7-10 minutes.
Brush the baguette with sauce. Cut the baguette into two halves, then cut each half lengthwise, make a “pocket,” and remove most of the pulp. Mix mayonnaise with mustard. Brush the sauce onto the bottom of the bread.
Add the filling. Place bell pepper strips and add salt to taste. Place shiitake with onions on top, sprinkle with grated cheese. Brush the top of the baguette with mustard-mayonnaise sauce and connect the halves. Place the baguettes on a heat-resistant baking sheet and bake for 10-15 minutes.
Place the baguette with shiitake mushrooms on a wide flat plate and sprinkle with chopped dill before serving.