Clean and thinly slice the onion.
Wipe the mushrooms with a wet cloth and slice them thin too.
Heat a tsp of oil in a pan, add in the sliced onions, sprinkle salt and allow them to sweat.
Add in the mushrooms a little at a time allowing them to firm a crisper texture than steaming all together. Adding a little at a time is important because we are adding only very little oil and the mushrooms love to soak up a lot of oil.
Once a semi dry texture is attained, add the tomato puree, ketchup. Saute well. Sprinkle on a bit of water and cover and cook for a few minutes.
Sprinkle pepper, salt.and oregano.
Cook for two to three minutes.
Take off the heat, add chopped basil leaves and set aside to cool