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Mix the red onion and lemon juice together in a small bowl, cover and set aside. Cook the bacon in a dry frying pan over a medium heat for 10-15 mins, turning halfway, until very crisp. Drain on kitchen paper, then crumble into tiny pieces and set aside.
To make the dressing, put all the ingredients in a bowl, season and whisk until well combined. Cut the lettuce into chunky pieces and place on a platter or in a bowl with the tomatoes, cucumber, radishes and onion. Just before serving, pour the dressing over and sprinkle with the bacon.