Heat olive oil in a large pot over medium-high heat. Add onion and cook 3 minutes, or until softened, stirring often. Add chili powder through paprika; stir to coat and cook 1 minute.
Add vinegar, broth, tomato sauce, and lentils. Bring to boil. Reduce heat to medium-low. Simmer partially covered for 30 minutes (or until lentils are tender), stirring frequently. Serve over whole wheat spaghetti and garnish with onion, cheese, and crackers.