Heat a large heavy-bottomed pot over medium heat and add the oil. Once shimmering, add the chopped onion and cook until it turns soft and golden, about 5-7 minutes, filling the kitchen with a sweet, nutty aroma.
Stir in the minced garlic and grated ginger, cooking until fragrant—about 30 seconds to 1 minute—until you smell a spicy, warm aroma filling the air.
Add the cumin, turmeric, and coriander, stirring constantly for about 30 seconds until the spices release their aroma and turn a slightly deeper color, creating a fragrant base.
Pour in the crushed or chopped tomatoes, stirring well. Let the mixture simmer gently for about 5 minutes until it darkens slightly and smells rich and savory.
Add the rinsed red lentils, stirring to coat them with the tomato-spice mixture. Then pour in the vegetable broth or water, bringing everything to a gentle boil.
Reduce the heat to low and let the curry simmer uncovered for about 20-25 minutes, stirring occasionally. The lentils will soften and begin to break down, thickening the curry and creating a velvety texture.
Once the lentils are tender and the curry has thickened, stir in the coconut milk for added creaminess, if using. Season with salt and pepper to taste, adjusting the acidity with a squeeze of lemon or lime juice.
If desired, stir in a handful of greens like spinach or kale and cook just until wilted, about 1-2 minutes, adding a fresh contrast to the warm spices.
Remove the curry from heat and let it rest for about 5 minutes to allow flavors to meld and deepen. Taste again and adjust seasoning if needed.
Scoop the hot, creamy lentil curry into bowls, garnishing with chopped herbs or a squeeze of citrus for brightness. Serve with rice, naan, or crusty bread and enjoy this comforting, nourishing dish.