Begin by rinsing the lentils thoroughly in cold water until the water runs clear, then set them aside to drain.
Chop the onion into small, even pieces and peel and slice the carrots into thin rounds.
Heat the olive oil in a medium pot over medium heat until it shimmers and begins to bubble gently.
Add the chopped onion to the pot and sauté, stirring frequently, until it turns golden and fragrant—about 5 minutes. You should smell a sweet, smoky aroma as the onions soften.
Add the sliced carrots to the pot and cook for another 3-4 minutes, stirring occasionally, until they begin to soften and release their aroma.
Stir in the minced garlic, cumin, and smoked paprika, cooking for about 30 seconds until the garlic becomes fragrant and slightly golden.
Pour in the rinsed lentils and vegetable broth, stirring well to combine all ingredients and scraping up any flavorful bits from the bottom of the pot.
Bring the mixture to a gentle boil over high heat, then reduce the heat to low, cover, and let simmer for 25 to 30 minutes. During this time, the lentils will soften and the flavors will meld, filling your kitchen with earthy, savory aromas.
Once the lentils are tender and the soup has thickened slightly, taste and season with salt and pepper as needed. For brightness, stir in a tablespoon of lemon juice or vinegar if desired.
If you like, stir in chopped greens like spinach or kale during the last 5 minutes of cooking for an extra touch of freshness and color.
Remove the pot from heat and let the soup sit for 5 minutes to allow flavors to deepen and settle. Then, give it a final taste and adjust seasoning if needed.
Serve the warm, hearty lentil soup in bowls, garnished with a squeeze of lemon or vinegar if desired, and enjoy this cozy, rustic dish with crusty bread or a side salad.