Heat the oil in a large pot over medium heat.
Add the mustard seeds, cover the pot, and cook for 1 to 2 minutes until the seeds begin to pop.
Add the diced onion, minced garlic, and ginger.
Sauté until the onion becomes soft and translucent.
Stir in the curry powder and garam masala.
Cook for another 1 to 2 minutes, stirring frequently to toast the spices and prevent them from scorching.
Add the rinsed lentils and water to the pot.
Bring the mixture to a boil, then reduce the heat to low, cover, and let it simmer for at least 60 minutes, or until the lentils are soft and tender.
Stir in the salt, pepper, and fresh lemon juice.
Taste and adjust the seasoning as needed before serving.