Rinse the red lentils under cold water until the water runs clear. Finely chop the onion, mince the garlic and ginger.

Heat the vegetable oil in a large pot over medium heat. Add the chopped onion and sauté until it becomes translucent, about 3-4 minutes. Add the minced garlic and ginger, and cook for another 1-2 minutes until fragrant.

Stir in the ground cumin, ground turmeric, ground coriander, garam masala, curry powder, and chili powder (if using). Cook for about 30 seconds to release the spices' aroma.

Add the rinsed red lentils to the pot, followed by the diced tomatoes and 4 cups of water or vegetable broth. Stir well to combine.

Bring the mixture to a boil, then reduce the heat to low and let it simmer uncovered for 15-20 minutes, or until the lentils are soft and the dahl has thickened. Stir occasionally to prevent sticking. Season with salt to taste.

Once the lentils are cooked, taste and adjust the seasoning if needed. Garnish with fresh chopped cilantro and serve with lemon wedges on the side.