In a large mixing jug, whisk the eggs. Cook the onions in a little oil until soft and remove from pan, putting in with the eggs. Fry the bacon until browned and remove from pan, also putting in with the eggs. Cook the tomatoes until browned, squeeze out some of the juice with the back of the spoon (if too much water is in the fritatta it wont set properly) and put the drained tomatoes into the eggs, mixing the fried ingredients into the egg. Put a little oil in a clean, good non stick pan and when the oil is hot enough, add the egg mixture, popping the cheese cubes into the egg all over. Cook on a low to medium heat until starting to set through to the middle. When you get to this point, place under a hot grill for 5 mins or so to finish cooking through. Slice into 6 wedges and serve...