Gather your crockpot, knife, cutting board, and wooden spoon to prepare for cooking.
Rinse the lentils thoroughly under cold water, picking out any debris or broken pieces, and set aside.
Chop the celery and carrots into small, uniform dice, about half an inch, and finely dice the onion. Mince the garlic cloves.
Add the diced onion, garlic, carrots, and celery into the crockpot. Drizzle with olive oil and stir gently so the vegetables are coated evenly.
Pour the vegetable broth into the crockpot, then add the rinsed lentils, ground cumin, dried thyme, and bay leaf. Give everything a gentle stir to combine all ingredients evenly.
Cover the crockpot with its lid and set it to cook on low for 6 to 8 hours, or on high for 3 to 4 hours. You'll notice the soup beginning to simmer and fill your kitchen with warm aromas.
During cooking, occasionally lift the lid to check the consistency and stir gently. The lentils should soften and start breaking apart, thickening the broth into a velvety texture.
Once the lentils are tender and the vegetables are fully softened, remove the bay leaf and squeeze in fresh lemon juice if using, to brighten the flavors.
Taste the soup and adjust seasoning with salt and pepper as needed, stirring to blend the flavors beautifully.
Ladle the hot, comforting soup into bowls, ensuring each serving is filled with tender lentils and flavorful broth. Garnish with fresh herbs if desired and enjoy immediately.