Finely chop the onion and the garlic. Heat some oil in a large pan and sauté the onion for 3-4 minutes until it's translucent.
Then add the ginger and the garlic and cook it for 30 seconds. Stir constantly. Then add the spices (cumin, curry powder, coriander, and garam masala) and cook for another 30 seconds until fragrant.
Add the red lentils, the coconut milk, the vegetable broth, and the diced tomatoes and stir well.
Let it simmer for 15-20 minutes without a lid. Stir a few times with wooden spoon during this time.
Season with salt, pepper, freshly squeezed lemon juice, and red pepper flakes. Skip the red pepper flakes for a milder version.
Serve with cooked rice, freshly chopped cilantro, and naan bread.