Heat the oil in a large pot or Dutch oven over medium-high heat. Add the carrots and cook for 3 minutes, stirring occasionally.
While the carrots are cooking, add the tomatoes, onion, jalapeño (if using), garlic, salt, black pepper, and Mexican oregano to a large blender. Blend for 2 minutes until completely smooth.
Add the blended tomato mixture, broth, chicken bouillon, dry lentils, potato, and cilantro to the pot. Stir to combine and raise the heat to high.
Bring to a boil, reduce heat to low, cover, and cook for 28 to 30 minutes until the lentils are soft and tender.
Remove the pot from the heat and discard the cilantro sprigs before serving.