Finely chop the mushrooms and eggplant.
Heat oil in a pan. Add chopped mushrooms, eggplant, minced garlic, thyme, pepper, and salt. Cook for around 12-15 minutes or until the moisture of the mushrooms get evaporated and the eggplants are also cooked. Switch off the gas and keep the mixture aside.
Melt margarine in a pan. Add flour and keep stirring until the flour is no longer raw. Add vegetable stock, 1/4 cup at a time while stirring continuously. Add nutmeg. Keep stirring and cooking until the mixture is smooth and thick.
Add the mushroom, eggplant filling. Mix. Check and adjust the seasoning. Switch off the gas and let the mixture reach room temperature. Transfer the filling to an airtight container and keep in the fridge for at least 4-5 hours or until the mixture solidifies.
Mix water and corn starch in a bowl to make a slurry. In two other separate bowls put flour and bread crumbs. Make an assembly line by first putting the flour bowl then corn starch slurry and finally the bread crumbs bowl.
Take a small portion of the mushroom filling in your hand. Roll between your palms to make a ball. Roll the ball first through flour then, corn starch slurry, and finally bread crumbs. Keep aside on a plate. Repeat the process with the remaining filling.
Put the balls in an airtight container and keep in the fridge for atleast 2-3 hours.
Deep fry the balls until they turn golden brown. Serve hot with mustard sauce.