Gather all your tools and ingredients: rinse the green lentils, dice the carrots and onion, and mince the garlic.
Warm the olive oil in a skillet over medium heat until it shimmers and starts to smell fragrant. Add the diced onions and carrots, sautéing until the onions turn translucent and the veggies begin to soften, about 5 minutes.
Transfer the sautéed vegetables to the slow cooker and add the rinsed lentils, diced tomatoes, vegetable broth, cumin, smoked paprika, and bay leaf. Stir everything together to combine well.
Cover the slow cooker with its lid and set it to low. Let it cook undisturbed for 6 to 8 hours, until the lentils are tender and the mixture is thick and fragrant.
About 30 minutes before the end of cooking, check the stew. If it appears too thick, stir in a splash more broth; if it’s too watery, remove the lid and cook on high for a bit to thicken.
Once the lentils are soft and the stew has thickened to your liking, turn off the slow cooker. Remove the bay leaf and stir in a squeeze of lemon juice if desired to brighten the flavors.
Taste the stew and adjust seasoning with salt and pepper. Ladle into bowls, garnish with fresh herbs if you like, and enjoy this cozy, thick lentil stew.