Slice around 3 cups of onions. Heat oil in a pan and add in the cumin seeds. Add in the sliced onions and the salt. Saute for 2-3 minutes till the onions are soft. No need to brown the onions.
Add in the red chilli powder and garam masala powder. Saute for a few minutes. The onions will be getting cooked again during baking. Add in the beaten rice (aval in tamil) and mix well. The aval will help absorb all the moisture from the onions so the samosa wrappers dont become soggy. Remove from heat and set aside to cool. The stuffing for baked onion samosa is ready.

Take a bowl and add in a cup of maida. Add in the oil, salt and the baking soda. The baking soda will help in browning and crisping up of the warappers. Mix well to combine. Add water little by little and mix to form a dough. The consistency of the dough should be stiff and that of a chapati dough. Knead the dough for a good 2-3 minutes so its soft and pliable. Divide the dough into 8 equal pieces. Shape the pieces into balls and set aside.
Take a dough ball and generously dust it with flour. Roll the dough into a 3 inch round on a flat smooth surface using a rolling pin. Brush oil on top of the rolled dough. Sprinkle flour on it. Place another rolled sheet on top of this and roll again. Sprinkle flour while rolling. It will expand. The thinner the sheet, the better. Dont press too hard. Be gentle while rolling. The oil and the sprinkled flour in between the sheets will help to remove the sheets later on.
Place the combined sheets on a hot griddle and cook for a few seconds on each side. No need to brown the sheets. Cooking for a few seconds till the moisture in the dough dries up is all we are looking for. We still need the sheets to be pliable. So do not cook for long. Remove the sheets from the pan and gently separate the sheets. Repeat the same with the remaining sheets. Cook all the sheets. Set aside.
Now using a knife, cut the sheets into four triangles. Cover them with a cloth so it does not dry up a lot. The triangle sheets are ready. Each triangle will make one samosa. In a small cup dilute three tablespoons of maida with two tablespoons of water. This is our maida gum. Set aside.
Take a triangle sheet. Fold on one side. Gather the other side and make a cone. Press well so the maida gum adheres. Fill the cone with the onion filling we made before. Apply the maida gum on the remaining sheet and fold to make a triangular samosa. Pinch the edges so there is no holes. Repeat with the rest of the sheets. Set aside.

Preheat the oven for 15 minutes at 200°C / 400°F. Take a greased sheet pan and place the prepared samosas. Brush the samosas with oil. Make sure to brush evenly so the entire samosa is evenly covered. Bake at 200°C / 400°F for 20-25 minutes. Bake in batches if necessary.
Half way through, after 10 minutes, remove the samosas from the oven and apply oil on the top again. Turn the samosas. The side that is in contact with the pan will brown faster. So turning will ensure that the samosas are baked evenly. Bake until the samosas are nice and brown.
Serve as a appetizer or as an evening snack.