Preheat the oven to 230 °C / 445 °F, Gas Mark 8, no fan (if you have a convection oven, reduce the temperature by about 20 degrees or follow the manufacturer‘s instructions).
Prepare the fish: cut the skin horizontally on one side in several places, season in the cuts and in the cavity of the trout with salt and pepper, place the lemon slices, lightly crushed garlic cloves and thyme sprigs inside the cavity.
Potatoes: Peel the potatoes, cut small potatoes in half, bigger ones in four - the pieces should be approximately 1 ½-inch / 3-4 cm thick.
Asparagus: Cut off the woody ends of the asparagus (where the green color turns into white).
Prepare a 12x17-inch sheet pan (or 34cm x 34 cm). Add potatoes and asparagus, toss them with olive oil, thyme sprigs, lemon zest, and garlic powder. Season to taste with salt and pepper.
Transfer the asparagus to a plate and spread the potatoes evenly on the baking tray. Bake the potatoes for 15-20 minutes.
Take the baking tray out of the oven, move the potatoes to the side of the pan and turn them over. I‘m using a thin metal spatula to do this, scraping the potatoes from the tray.
Place the asparagus and fish on the baking tray, bake for about 8-10 minutes. The baking time will depend on the size/thickness of your fish. The fish is ready when the flesh turns white and the asparagus should be tender but still crunchy. It’s important not to overbake the fish because it will be tough and dry.
Prepare the thyme-garlic yogurt dip: mix all the ingredients in a small bowl together, serve with the fish, potatoes, and asparagus.