Heat the oil in a large pot over medium heat until shimmering and fragrant.
Add the chopped onion and cook, stirring occasionally, until translucent and soft, about 5 minutes. The onion should be glossy and fragrant.
Stir in the minced garlic, cumin, turmeric, and chili powder. Cook for about 30 seconds to 1 minute until the spices release their aroma and the mixture is fragrant.
Add the rinsed red lentils and diced tomatoes to the pot. Pour in 3 to 4 cups water or broth, enough to cover the lentils. Stir to combine.
Bring the mixture to a boil over high heat, then reduce the heat to low and cover partially. Let it simmer gently for 20-25 minutes, stirring occasionally, until the lentils are tender and starting to break apart.
Once the lentils are soft and the mixture has thickened, stir in the coconut milk. Season with salt and pepper to taste, then continue to simmer uncovered for another 5 minutes until the curry is creamy and well combined.
Check the seasoning and adjust salt or chili as desired. The curry should be thick, glossy, and fragrant, with the lentils fully broken down into a smooth, hearty sauce.