In a 4-quart saucepan, heat oil on medium until hot. Add carrots and onion, and cook 6-8 minutes or until lightly browned and tender. Stir in cumin; cook 1 minute.
Add tomatoes, broth, lentils, 2 cups water, salt, and pepper. Cover and heat to boiling on high. Reduce heat to low and simmer, covered, until lentils are tender. Stir in spinach.