The day before required, thinly slice the onions into rings and place in a deep bowl. Combine the water and sugar in a saucepan, bring to the boil then pour onto the onion rings. Immediately cover the surface of the onions with ice cubes and place in the refrigerator. In a bowl mix the mayonnaise, mustard and lemon juice. Refrigerate until required. The next day, drain the onion rings and dry on paper towels. Spread the mayonnaise mix on the number of bread slices required and top with onion rings. Season with salt and freshly ground black pepper and top each with another slice spread with the mayonnaise. Trim off the crusts and cut into triangles. Store in the refrigerator if not required immediately, or beneath a cool, damp tea towel. SIMON SAYS: Refrigerate onions before chopping them to reduce the tears. Rubbing your hands with vinegar before and after slicing onions will eliminate any lingering smell.