Heat olive oil in a 5-quart / 4.7 L dutch oven or soup pot over medium heat. Add the onion, carrots, and celery, and cook for 6 to 7 minutes until softened.
Stir in the garlic, cumin, smoked paprika, and thyme. Cook for 1 minute until fragrant.
Add the rinsed lentils, diced tomatoes, broth, and bay leaf. Bring to a boil over high heat.
Reduce heat to a low simmer and cook uncovered for 25 to 30 minutes, stirring occasionally, until the lentils are tender but still hold their shape. Check at 25 minutes, they should not turn mushy.
Remove the bay leaf. Stir in the spinach and cook for 2 to 3 minutes until just wilted.
Stir in the lemon juice, salt, and pepper. Taste and adjust seasoning.
Ladle into bowls and serve warm, with bread on the side if you like.