Heat water in an electric tea pot for both boiling the sweet potato and soaking the soya. While water is heating, dice the sweet potato to decrease cooking time. Soak soya chunks in hot water and begin boiling sweet potatoes.
Slice the red onion and dice one tomato. The second tomato should be sliced and set aside for topping the sandwich. Heat the oil in a frying pan over medium high heat.
Add the sliced onion and diced tomato to the frying pan. Sauté for 3-5 minutes or until the tomatoes are beginning to lose their structure. Add the drained soya chunks to the pan, reserving ¼-½ cup soaking water.
Add the cumin, turmeric and salt. Stir until all ingredients are mixed well. Cover and let simmer for 3-5minutes, stirring occasionally.
Drain the sweet potatoes and place in a bowl with the avocado. Using a fork and spoon, mash the two with salt until creamy. Spread on the bottom of a toasted bun.
Add the soya chunks on top of the spread covered bun. Then add sliced tomatoes. Top with the second half of the bun. Enjoy!