Heat the olive oil in a large pot or Dutch oven over medium heat. Add the finely chopped yellow onion, diced carrots, and diced celery. Sauté the vegetables, stirring occasionally, until they begin to soften, which should take about 8-10 minutes. Be careful not to brown them too much; you want them tender.

Add the minced garlic, smoked paprika, ground cumin, dried thyme, and optional crushed red pepper flakes to the pot. Stir constantly for about 1 minute until fragrant. This step toasts the spices, releasing their full aroma and flavor. Be sure not to burn the garlic.

Stir in the tomato paste and cook for another 1-2 minutes, stirring constantly. This helps to deepen the tomato flavor and remove any raw taste from the paste.

Pour in the undrained diced tomatoes and the vegetable broth. Scrape the bottom of the pot with a wooden spoon to loosen any browned bits, which will add extra flavor to the stew.

Add the rinsed lentils and the bay leaf to the pot. Bring the mixture to a boil, then reduce the heat to low, cover the pot, and let it simmer. Cook for 30-40 minutes, or until the lentils are tender but not mushy. Stir occasionally to prevent sticking.

Once the lentils are tender, remove and discard the bay leaf. Season the stew generously with salt and freshly ground black pepper to taste. Stir well to ensure the seasoning is evenly distributed.

Ladle the hot stew into bowls. Garnish generously with fresh chopped parsley. Serve immediately, optionally with crusty bread for dipping.