Whisk together eggs, milk, salt, chopped onion, garlic and black pepper in a bowl. Add the flaked tuna and whisk lightly.
Melt the margarine in an omelette pan over medium flame; spread evenly over the base of the pan.
Pour in the egg mixture and cook to the point at which soft curds begin to form.
Spread on half the diced green and yellow sweet peppers and half the tomatoes.
Grate on the cheese and ensure it is distributed evenly; cover for about 30 seconds.
Remove the lid and use an egg lifter or spatula to fold it in half, completely encasing the filling.
Garnish with the remaining diced vegetables.
To Serve: Serve on a heated plate with Toast and a Fruit Plate.