In a medium saucepan or pot, heat the canola oil over low heat. Add the onions and cook until translucent, about 2-3 minutes.
Add the red curry paste and stir into the onions for about a minute, until fragrant. Pour in the chicken broth, coconut milk, and fish sauce and increase the heat to medium-high, and bring to a boil. Let it cook, stirring occasionally for about 5 minutes.
Add the sweet potatoes and kale. Cook for about 5-10 minutes, or until the sweet potatoes are fork-tender, taking care to not let them get mushy. Adjust seasoning with salt and pepper if necessary.
Serve the soup while hot, garnishing with cilantro and creme fraiche.