In a large pot over medium-high, combine the oil, onion, garlic and 1 teaspoon salt. When you hear a sizzle, cover the pot, turn heat to low, and cook, stirring every so often, until the onion has softened and is just beginning to color, 7 to 9 minutes. Stir in the ginger, the mustard seeds, turmeric, coriander, fennel and red pepper flakes. Cook, stirring often, until fragrant, about 1 minute.

Add the water, coconut milk, lentils, and remaining half teaspoon salt, then bring to a boil.

Reduce to low, cover and simmer, stirring once or twice, until the lentils have broken down, 40 to 50 minutes.

Stir in the spinach and return to a simmer.
Taste and season with salt if desired.
Serve immediately.