Grill (broil) or fry the bacon until the rind is crisp. Drain on absorbent paper, then chop them coarsely. Remove the crusts from the bread and butter them lightly. Divide the chopped bacon equally between the 4 slices and sprinkle chopped spring onion (scallion) on each slice. Season with black pepper, then make up the sandwiches in the normal way. Cut each sandwich into 4 triangles. Beat the eggs and tomato puree together. Add the coriander and salt and pepper to taste. In a frying pan, heat the remaining butter and the oil together over a medium heat. Dip each sandwich in the egg mixture and fry until golden brown on both sides. Drain on absorbent paper and serve immediately.