Wash and dry all produce. Preheat oven to 425 degrees. Peel carrot, then slice on diagonal into 1/4-inch pieces. Trim and halve Brussels sprouts lengthwise. Toss veggies on a baking sheet with 1 TBSP olive oil and a pinch of salt and pepper (leave room for salmon). Roast until lightly browned and softened, about 20 minutes, tossing halfway through.

Place lentils in a medium pot with thyme, stock concentrate, and enough water to cover by 1 inch. Bring to a boil, then reduce to a simmer and cook until tender, about 25 minutes. Drain, discard thyme sprig, and set aside.

While veggies and lentils cook, halve, peel, and mince 1 TBSP shallot. Halve lemon. Put mustard, shallot, and juice of half the lemon in a large bowl. Whisk to combine and season with salt and pepper. Whisk in 1 TBSP olive oil and set aside.

With 10 minutes to go on veggies, add salmon to same baking sheet. Season with salt and pepper. Roast until opaque and just cooked through, about another 10 minutes.

Season cooked lentils with salt and pepper. Add to bowl with dressing and toss. Add carrots and Brussels sprouts and toss to combine.

Divide lentil salad between plates. Top with salmon and squeeze lemon juice (to taste) over everything.