Heat the olive oil over medium heat in a large, heavy stock pot. Add the onion and garlic and sauté until onion is translucent. Stir in the ginger, cardamom pods, coriander, red pepper flakes, and salt. Sauté for one more minute. Add the vegetable stock or water, then add the cubed sweet potatoes. Bring to a boil, then reduce heat to low, and simmer for 10 minutes. Add the tempeh, and simmer for 8-10 minutes, until the tempeh is cooked through and the sweet potatoes are tender.
Taste, and adjust seasonings if needed. Stir in 1/2 cup of the yogurt, then serve. Top each bowl with a dollop more of yogurt and a sprinkle of the toasted pumpkin seeds.