Prep the ingredients: Preheat the oven to 400 degrees. Halve and slice the potatoes into 1⁄2-inch wedges. Halve, peel, and chop the onion into 1-inch cubes. Strip the thyme from the stems and roughly chop the leaves.
Roast the potatoes and onions: Toss the potatoes and onions on a baking sheet with the thyme, a drizzle of olive oil, and a pinch of salt and pepper. Place in the oven for about 20 minutes, tossing halfway through, until golden brown.
Cook the lentils: Place the lentils in a small pot with the stock concentrate and enough water to cover by 1-inch. Bring to a boil, then reduce to a simmer for 20 minutes, until tender. Drain and season with salt and pepper.
Roast the beets: Meanwhile, cut the beets into 1⁄2-inch wedges. In a large bowl, whisk together 1 Tablespoon balsamic and 1⁄2 Tablespoon honey. Toss in the beets and season with salt and pepper. When the potatoes and onions are almost done, add the beets to the baking sheet and cook for 5 minutes.
Combine the salad: Once the roasted veggies are done, toss them 5 in the same large bowl with the warm lentils, a large drizzle of olive oil, and the remaining balsamic. Carefully toss in the arugula and season with salt and pepper.
Plate: Serve the salad sprinkled with the feta cheese and walnuts, and enjoy!