In either a pot on the stove over medium heat or in an Instant Pot on the sauté setting, heat the oil. Add the onion and cook while stirring until it softens, 3 - 4 minutes.
Add the garlic, sweet potatoes, lentils, and spices and cook while stirring for another minute or two.
Pour in the diced tomatoes and broth. Stovetop: Bring to a boil then reduce to a simmer until the potatoes and lentils are tender, about 15 minutes. Instant Pot: Secure the lid, make sure the valve is switched to "sealing," push the "soup" button, and reduce the time to 3 minutes using the "-" button. Once the Instant Pot is finished counting down, push the "cancel" button and wait for the steam to release naturally until the float valve goes all the way back down before opening the lid.
Ladle into bowls, top with a dollop of plain yogurt and some fresh cilantro leaves, and serve warm.