Prep the ingredients: Preheat the oven to 400 degrees. Trim and halve the Brussels sprouts lengthwise. Thinly slice the mushrooms. Thinly slice the garlic. Halve, peel, and finely dice the shallot. Thinly slice the lemon into rounds. Finely chop the parsley, reserving a few leaves for garnish.

Roast the Brussels sprouts: Toss the Brussels sprouts on one side of a baking sheet with a drizzle of oil and a pinch of salt and pepper. Place in the oven for 20-25 minutes, until golden brown and tender.

Assemble the papillote: Lay out a large piece of tin foil and place a drizzle of oil into the center of the foil. Season the trout on all sides with salt and pepper, then place 1 fillet into the center of the foil. Top with a lemon round and a sprig of rosemary, then gather the edges of the foil to seal into a package. Repeat with the remaining fillet. Place packages onto the baking sheet with the Brussels sprouts and return to the oven for about 15 minutes, until Brussels sprouts are golden brown and fish is opaque.

Cook the vegetables: Heat a drizzle of oil in a large pan over medium-high heat. Add the shallots, garlic, and shiitake mushrooms to the pan and cook, tossing, 4-5 minutes, until softened and slightly golden brown. Set aside.

Make the mushroom-pan sauce: Heat 2 tablespoons butter in the same pan over medium heat. Once melted, whisk the flour into the pan and cook, whisking constantly, for 1 minute. Very gradually add 1 cup water, whisking constantly, until combined. Add the stock concentrate, parsley, and the mushroom mixture to the pan and stir to combine. Bring to a boil, then simmer for 3-4 minutes, until thickened.

Finish: When the trout is ready, remove it from the foil and plate alongside the roasted Brussels sprouts. Spoon the mushroom-pan sauce over the fish, then garnish with the reserved parsley leaves and a lemon round. Enjoy!