Mix all marinade ingredients together.
Cover the mushrooms with marinade on both sides.
Cook mushrooms in the frying pan covered with a tight lid, on medium-low heat, for about 4 minutes on each side.
Cooking time may vary depending on a size of a mushroom. If your mushrooms are small, it will need 3 minutes of each side.
While mushrooms are cooking, toast the buns slightly in the oven. You may choose to skip this step.
Spread 1 tsp of mustard on each bun.
Once the mushrooms are cooked, lift the lid, place the cheese on top, and return the lid, allowing cheese to melt.
Transfer the mushrooms onto the buns, top with fried onions and assemble the burger.