Place the lentils in a fine-mesh colander and rinse under cold running water until the water runs clear.

Heat the olive oil in a large heavy-bottomed pot over medium heat until shimmering, about 1 minute.

Add the diced onion, carrots, and celery and sauté for 5 to 7 minutes, stirring occasionally, until softened.

Stir in the minced garlic, cumin, and paprika and cook for 1 minute until fragrant.

Add the rinsed lentils and vegetable broth, stirring to combine and ensuring the lentils are covered by about an inch of liquid.

Stir in the diced tomatoes with their juice, bay leaf, sea salt, black pepper, and red pepper flakes if using.

Increase heat to medium-high and bring the soup to a boil, then reduce heat to low and simmer uncovered for 25 to 30 minutes, stirring occasionally.

Remove and discard the bay leaf, stir in the fresh lemon juice, and adjust seasoning with additional salt and pepper to taste before serving.