Chop the onion into small dice and mince the garlic. Slice the carrots thinly and rinse the lentils thoroughly. Measure out all ingredients to prepare for cooking.
Heat the olive oil in a sauté pan over medium heat until it shimmers and begins to gently bubble. Add the diced onion and cook, stirring occasionally, until translucent and fragrant—about 5 minutes. Next, stir in the minced garlic and cook for another minute until the aroma is inviting and the garlic is slightly golden.
Transfer the sautéed onions and garlic to the slow cooker. Add the sliced carrots, rinsed lentils, canned diced tomatoes (with juices), vegetable broth, cumin, smoked paprika, and chili flakes if using. Stir everything together until well combined.
Cover the slow cooker and set it to low for 6 to 8 hours, or on high for 3 to 4 hours. During cooking, you'll notice the lentils and vegetables softening, and the broth thickening into a hearty stew with a rich aroma wafting through your kitchen.
About halfway through, give the stew a gentle stir to ensure even cooking and prevent sticking. If the stew is thickening too much before the lentils are tender, add a splash more broth or water. When the cooking time is complete, check that the lentils are tender and the vegetables are soft.
Taste the stew and adjust the seasoning if needed—adding a pinch of salt or more spices to enhance the flavor. For a creamier texture, mash some of the lentils with the back of a spoon directly in the pot.
Turn off the slow cooker and let the stew rest for about 10 minutes. This allows the flavors to meld beautifully. Serve hot, garnished with fresh herbs or a squeeze of lemon if desired, for a bright finish.
