Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper.

Toss diced beetroot in 1 tablespoon olive oil, salt, and pepper. Spread on one side of the baking sheet.

Roast beetroots for about 20 minutes until tender, stirring halfway through.

While beetroots roast, rinse lentils. Cook in a pot with enough water to cover until tender (about 15-20 minutes). Drain excess water.

In a small bowl, mix Dijon mustard with remaining olive oil, salt, and pepper. Brush onto both sides of salmon fillets.

After roasting beets for 20 minutes, push them aside on the baking sheet and place salmon fillets next to them. Bake for another 12-15 minutes until salmon is cooked through.

Serve salmon alongside lentils and roasted beets, drizzled with any leftover mustard sauce.