Rinse the lentils under cold water and drain.
In a large pot, bring 2 cups of water to a boil. Add the lentils, reduce heat to low, cover, and simmer for about 15-20 minutes, or until the lentils are tender but still hold their shape. Drain any excess water and set aside.
In the same pot, heat the vegetable oil over medium heat. Add the chopped onion and cook until softened, about 5 minutes.
Add the minced garlic, curry powder, ground cumin, ground coriander, turmeric, and cayenne pepper (if using). Cook, stirring constantly, for about 1 minute, until fragrant.
Stir in the diced tomatoes (with their juices) and coconut milk. Bring the mixture to a simmer.
Add the cooked lentils and chopped mixed vegetables to the pot. Stir well to combine.
Cover the pot and simmer for about 15-20 minutes, or until the vegetables are tender.
Season the curry with salt and pepper to taste.
Serve the lentil and vegetable curry hot, garnished with fresh cilantro if desired. Enjoy with cooked rice or naan bread.