In a large skillet, heat the coconut oil over medium heat. Once melted, add the chopped onions and sauté until they become translucent, about 4-5 minutes. Add the minced garlic and cook for an additional minute, stirring frequently to prevent burning.
Next, add the lentils to the skillet and stir to combine with the onions and garlic. Cook for about 1-2 minutes to let the lentils absorb the flavors.
Pour in the two cans of reduced-fat coconut milk, stirring well to combine. Bring the mixture to a gentle simmer, and allow it to cook for about 20 minutes, or until the lentils are tender.
Once the lentils are soft, stir in the natural peanut butter, coconut sugar, paprika, fish sauce, lime juice, red pepper flakes, and optional salt. Mix thoroughly until the peanut butter is fully incorporated.
Toss in the fresh spinach, stirring until it wilts down and brightens in color. Let the curry simmer for another 5 minutes to allow all the flavors to meld together.
Adjust the heat with sriracha according to your spice preference. Taste the curry and adjust seasoning if necessary. If you want more acidity, a little extra lime juice can go a long way.