Brush a bit of oil on the pitas and cut them into strips; spread out on a baking sheet and bake at 350F for about 10 minutes, or until golden. Set aside.
In a large saucepan, cover the lentils with enough water to cover by 2 inches. Add tomato, potato, carrot, and salt and bring to a boil; reduce heat and simmer for 30 minutes, skimming any foam that forms on top. Meanwhile, heat a drizzle of oil in a medium pot set over medium-high heat and sauté the onion for about 8 minutes, or until golden.
Remove the lentil mixture from heat and purée using a hand-held immersion blender or in batches in the regular blender. Add to the onions along with the beef stock and cumin. Bring to a simmer and cook for about 10 minutes, until thickened. Season with salt and pepper to taste and serve immediately, topped with the pita strips and parsley or cilantro.