Preheat oven to 400 F (200 C).
Spray a 9x13 baking pan with non-stick cooking spray.
Heat 1/4 cup oil in a large frying pan.
Saute onions until transparent. Set aside to cool.
In a large bowl, mix the rest of the oil with eggs and water.
Add cooled sauteed onions.
In a small bowl, mix flour with baking powder. Stir into the onion-egg mixture.
Spice with onion soup mix and pepper.
Add mushrooms, if desired.
Spoon mixture into the greased pan.
Bake, uncovered, at 400 F (200 C) for 45 to 60 minutes, or until golden on top. Baking times depends on the size of the pan used (smaller pans with thicker kugels require more baking time).