Cut the fish into large cubes, put it into a dish and sprinkle with lemon juice, salt and pepper. Cut the onions into wedges, put them into a separate bowl with the mushrooms, add a good grinding of pepper, and oil and lemon juice in the ratio of 2 tablespoons oil to 1 tablespoon lemon. Leave to absorb flavours for an hour or more. Halve the bacon rashers, de-rind them, stretch and roll them up loosely. The sauce can also be made in advance. Beat the honey, mustard and egg yolk together in a small bowl. Add the vinegar, a spoonful at a time, whisking it in with a balloon whisk or fork. Then add the oil, pouring it from a jug in a thin stream and whisking the contents of the bowl all the time as you pour. Continue whisking for about a minute until the sauce is a homogeneous thick cream, then season to taste with salt and pepper. Close to serving time, thread the fish, onions and mushrooms on to skewers, adding the bacon rolls and tomatoes here and there. Grill for 10 minutes or so under medium-high heat, turning and basting occasionally with the marinade liquid. Serve immediately with a dish of rice and hand round the sauce separately.