Roast the beets. Preheat oven to 400°F (200°C). Toss whole beets lightly with 1 tbsp olive oil and a pinch of salt. Wrap in foil or place on a rimmed baking sheet. Roast 30–40 minutes, until a knife slides in easily. Let cool, peel (the skin slips off), then dice into 1/2-inch cubes.
Cook the lentils. While beets roast, put lentils in a saucepan with 3 cups water and a pinch of salt. Bring to a simmer, then cook gently 18–22 minutes until tender but not mushy. Drain and let cool slightly. (Tip: brown or green lentils hold shape best.)
Make the dressing. Whisk together 2 tbsp olive oil, lemon juice, Dijon, garlic (if using), and salt & pepper. Taste and adjust acid/salt.
Assemble. In a large bowl, combine warm or room-temperature lentils, roasted beet cubes, chopped parsley (and mint if using). Drizzle dressing and toss gently to coat. Fold in toasted walnuts or pepitas. If using feta, sprinkle on top.
Rest or serve. Let the salad sit 10 minutes if possible — the flavors meld. Serve warm, room temp, or chilled.
