Heat the oil in a large heavy-bottomed pot over medium heat. Once shimmering, add the chopped onion and cook, stirring occasionally, until translucent and fragrant, about 5 minutes.
Add the minced garlic and grated ginger to the pot, stirring until fragrant, about 1 minute. The mixture should smell aromatic and slightly sizzling.
Stir in the cumin seeds (or ground cumin), turmeric, and chili powder if using. Toast the spices for about 30 seconds until they release a warm, nutty aroma and turn slightly darker.
Add the rinsed red lentils to the pot, stirring well to coat them with the spice mixture. Cook for 1-2 minutes, allowing the lentils to absorb the flavors.
Pour in the vegetable broth and diced tomatoes (if using). Bring the mixture to a boil, then reduce the heat to low, cover the pot, and let it simmer gently for 20-25 minutes. Stir occasionally to prevent sticking.
Check the lentils for tenderness—they should be soft and breaking apart, creating a creamy consistency. If the curry is too thick, stir in a splash of hot water or broth to loosen it.
Season the curry with salt to taste, stirring well. Let it sit off the heat for 10 minutes, allowing the flavors to meld further.
Give the curry a final stir, then serve hot with rice, flatbread, or your favorite accompaniment. Garnish with fresh herbs if desired for an extra touch of brightness.