Filling: In a large skillet heat up oil over high. Lower temperature to medium and add onion. Cook until translucent (3-5 min). Raise temperature to high again and add mushrooms and salt. Saute, and frequently stir mushrooms until they’re tender and the liquid they expel has mostly evaporated (15-20 min). Set mushroom crepe filling aside.
Crepes (palačinke): (Making palačinke can be done parallel to making the crepe mushroom filling.) In a large bowl combine flour, eggs, salt, milk and baking powder. Mix batter with a hand mixer. Slowly add water to batter as you mix. Pause mixing once or twice and scrape batter from the sides of the bowl. Crepe batter is ready when it’s smooth, crumb-free, and has the consistency of heavy cream. (You may have to add a little bit more or a little bit less water.)
Take a piece of paper towel, wrinkle it up in your palm, and dip it in oil. Use this to carefully coat the crepe pan. Heat the coated pan up on highest until the oil is really hot (2-3 min). The oil may get slightly smokey.
Grab crepe batter with a ladle (or another tool with a volume of approximately ½ cup) and transfer to a crepe pan (sized approximately 8-9 inches in diameter). Lift the pan and watching that the batter doesn’t leave the pan, swirl it around until it coats the pan bottom evenly and there are no ‘holes.’ Return to the stove and cook the crepe 60-90 seconds on one side. Flip to the other side and cook another 60 seconds. Transfer the finished crepe onto a plate.
Repeat step 4 until you run out of crepe batter. You should have 15-16 crepes, and it’ll take about 30-40 min to cook them. Halfway through cooking the crepes (palačinke) heat oven to 350°f (175°c). Then place butter cubes into a 10x15 inch baking pan and heat it up in the oven for 3 min, or until butter melts.
Assembly: (Assembling/ stuffing the crepes with mushroom filling can be done parallel to making the crepes.) Fill each crepe with a heaping tablespoon of mushroom crepe filling. Roll them up and place them in the hot pan. Repeat until you run out of crepes and filling. Pour and spread tartar sauce evenly over the crepes. Bake (15-20 min), until crepes and tartar sauce congeal and get a blush a little bit.
Serve crepes warm with (optional) additional tartar sauce. Keep in the fridge for up to a day. They can be consumed cold or warm.