Preheat oven grill to 220 °C.
Beat eggs with milk, baking powder, salt and pepper and gently fold in the soda water.
In a non-stick pan with a metal handle, saute mushrooms, lounza and onions in butter and olive oil, on medium heat.
When browned, add egg mixture (do not mix) and leave for 7 minutes.
Remove from heat and add the halloumi, tomatoes and spring onions.
Place under the grill for approximately 7 minutes or until the omelette rises and becomes fluffy.
Carefully remove and shake pan to loosen.
Slip onto a platter and serve in slices.

*Pan handle must be metal as we place the pan onto the oven.
This is a great option for a tea party or cocktail served on a baguette.
Get creative with your toppings!