Heat the olive oil in a large pot over medium heat. Add the onion and cook for 4 to 5 minutes, until softened.
Stir in the garlic, cumin, smoked paprika, and red pepper flakes, and cook for 30 seconds until fragrant.
Add the lentils, diced tomatoes with their juices, vegetable broth, and bay leaf. Bring to a boil, then reduce the heat and simmer uncovered for 25 to 30 minutes, stirring occasionally, until the lentils are tender.
Remove the bay leaf, then stir in the spinach and cook for 1 to 2 minutes until wilted.
Stir in the lemon juice, salt, and black pepper. Taste and adjust seasoning as needed before serving.