Heat the oil in a large pot over medium heat. Add the onion and a pinch of salt and cook until soft and lightly browned around the edges, 8 to 10 minutes, reducing the heat to low as needed.

With the heat on low, add the garlic, ginger, curry powder, and red pepper flakes and cook, stirring, until fragrant, about 2 minutes.

Add the tomatoes, lentils, water, coconut milk, ½ teaspoon salt, and several grinds of black pepper. Bring to a boil, cover, and reduce to a simmer, stirring only occasionally, until the lentils are tender, 25 to 35 minutes. If your soup is too thick, stir in 1/2 to 1 cup more water to reach your desired consistency.

Stir in the cilantro and lime juice. Season to taste with salt and pepper and serve.

Store leftover soup in the fridge for up to 4 days. If it thickens too much in the fridge, stir in a little water while you reheat it until you reach your desired soup consistency. This soup also freezes well.