Step 1

Heat oil in a large saucepan over medium. Add onion, garlic, and ginger; season with salt. Cook, stirring often, until onion is translucent and starts to soften, about 3 minutes. Add curry paste and cook, stirring and scraping bottom of pan constantly, until paste is fragrant and slightly darkened and mixture starts to stick to pan, about 3 minutes.

Step 2

Stir sweet potato and lentils into onion mixture, then add broth and 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt and bring to a boil. Reduce heat to a simmer and cook soup, stirring occasionally, until sweet potatoes are cooked through and lentils are tender but not mushy, 20–25 minutes.

Step 3

Add coconut milk to soup; return to a simmer. Add spinach and fish sauce and cook just until spinach is wilted, about 30 seconds. Taste soup and season with more salt and/or fish sauce if needed.

Step 4

Ladle soup into bowls and top with cilantro. Serve with lime wedges.