Cook the potatoes
Cut the new potatoes in quarters and add them to a pot filled with salted water. Place over high heat and cook until the water starts to boil, then lower the heat and cook for 5 minutes. Drain the potatoes in a colander and leave them for about 10 - 15 minutes for the steam to release. Preheat the oven to 220 °C / 430 °F or fan-assisted oven to 210 °C / 410 °F.
Roast the potatoes
Line your sheet pan with parchment paper. Add the drained potatoes. Drizzle them with three tablespoons of olive oil and season with cayenne pepper and black pepper. Toss to combine, then place in the oven on the middle rack. Roast for 20 minutes at 220 °C / 430 °F or fan-assisted oven at 210 °C / 410 °F.
Herb dipping sauce
Prepare the Herb dipping sauce. In a small bowl, stir to combine sour cream, chopped parsley, one tablespoon of olive oil, 1/2 tsp of salt, 1/2 tsp of black pepper, and tabasco. Set aside until needed.
Sheet Pan Trout Asparagus and Potatoes
Remove the potatoes from the oven. Using a spatula, push the potatoes on one side of the sheet pan, arrange the asparagus in the middle and place the trout fillet on the opposite side of the potatoes. Season the trout with salt and pepper. Place two lemon slices on top and drizzle everything with one tablespoon of olive oil. Return to the oven and roast for another 5 - 8 minutes at 240 °C / 465 °F or fan-assisted oven at 220 °C / 430 °F or until the fish separates easily with the fork. If necessary, season to taste with salt, then transfer to a serving platter and serve with the dipping sauce.
Remove from the oven and serve with asparagus and herb dipping sauce served on the side.