Spoon the yogurt and mayonnaise into a mixing bowl. Add the vinegar, dill and lemon zest and season well with salt and pepper. Taste, adding more vinegar to sharpen if you like.
Add the cucumber, onion and tomatoes and mix gently. Cover and chill until needed, up to 2 hours ahead.
Make the trout
Fire up the barbecue for direct grilling.
Prepare the trout by slicing deep diagonal cuts through the skin on both sides. Stuff the parsley stems into the body cavity of each fish and drizzle a little oil over each one. Season on both sides with salt and pepper.
If you are using a grilling basket, make sure your grill bars are hot and clean them first using a wire brush to minimize sticking. Coat the grill basket with cooking spray or oil, then line up the fish in the grilling basket.If you are cooking the fish directly on the grill, oil the preheated grill well.
Grill the trout, turning carefully, until cooked through and the skin is crisp and the internal temperature is 140°F (60°C), about 6 minutes per side. Rest the lemon halves on the grill at the same time, cut-side down, until the surface lightly burns and caramelizes, about 5 minutes.
☞ TESTER TIP: Depending on the size of your fish, your grilling time may need to be adjusted.
Serve the fish with the cucumber salad alongside and the grilled lemons to squeeze over.