Chop red and green bell peppers, set aside.
Heat half the vegetable oil in a small frying pan over medium heat, add half the butter.
When melted, add onions and mushroom, season with a little salt and pepper.
Stir and fry until the mushrooms are soft and brown.
Add red and green peppers, continue frying until softened.
Remove from the pan and set aside.
Crack eggs in a bowl, add a pinch of salt and lightly beat the eggs.
Wipe frying pan and put back over the flame.
Add the remaining vegetable oil and butter, when hot, pour the beaten eggs into it.
Pull the egg in from the sides as it fries and stop when it begins to set.
When almost set, spread mushrooms on one-half and leave to fry for a minute or two until set.
Lift one side of the frying pan and fold the empty half of the omelette to cover the mushrooms.
Slide onto a plate. Serve with a green salad.