In a pot over medium-high heat, add 1 tablespoon of the water, onion, sea salt, oregano, cumin, cinnamon, mustard, and turmeric.
Stir through, cover, and cook for 4–5 minutes (keep an eye on it, and add another splash of water if the spices and onion are sticking).
Remove the cover and add the lentils and remaining 3 1/2 cups water.
Turn heat up to high, and bring the mixture to a boil.
Once at a boil, reduce heat to low, cover, and cook for about 15 minutes.
After 12–15 minutes, add the apple and lemon juice, stir through, and cook for another 5–7 minutes, until the apple has softened a little but isn’t entirely mushy.
Season with additional sea salt or lemon juice if desired, and serve.