Wash and dry produce. Halve, peel, and dice onion into ½-inch pieces. Trim, peel, and halve carrots lengthwise; slice crosswise into ¼-inch half-moons. Halve celery lengthwise; thinly slice crosswise into pieces. Drain and rinse lentils.

In a medium pot (large pot for 4 servings), heat a large drizzle of oil over medium-high heat. Add onion, carrots, celery, salt, and pepper; cook, stirring occasionally, until veggies are slightly softened, 2-3 minutes.

Add garlic powder, chili powder, cumin, and tomato paste; cook, stirring, until fragrant, 30-60 seconds.

Stir in 2½ cups water (5 cups for 4 servings), lentils, and stock concentrates, scraping up any browned bits from bottom of pot. Bring to a simmer; cook, uncovered, until slightly reduced, 5-7 minutes.

Meanwhile, empty kidney beans and their liquid into a medium bowl. Mash with a potato masher or fork until beans are mostly smooth (this will help thicken the soup).

Once soup has simmered 5-7 minutes, stir mashed beans into soup. Return to a boil, then reduce to a simmer. Cook, stirring occasionally, until soup thickens, 3-5 minutes more. Taste and season with salt and pepper if desired.

While soup simmers, thinly slice chives.

Divide soup between bowls. Dollop with sour cream and sprinkle with chives. Serve.