In a large heavy-bottomed pot, heat olive oil and sauté the onion for a few minutes. Add the cinnamon and chilli powder and continue to sauté for another 2 minutes. Mix in the garlic and cumin seeds, sauté for 2 minutes more before adding lemon juice and the tomatoes. Mix thoroughly so the stew doesn't get too chunky.
Stir the pumpkin, pinto beans and water into the stew. Season with salt and pepper to taste. Let the stew simmer for 1 hour, or until pumpkin is tender. Stir occasionally and add more water if necessary. The finished stew should have a nice, thick texture.