Heat the saucepan over medium heat and add the oil, letting it shimmer slightly as it warms.
Add the chopped onion and minced garlic to the pan, sautéing until fragrant and golden, about 5 minutes. You should hear a gentle sizzle and see the onions turn translucent.
Stir in the turmeric, cumin, and coriander, cooking for about 30 seconds until the spices release their warm aroma.
Add the diced tomatoes and cook until they break down and the mixture darkens slightly, about 5 minutes. The smell should become tangy and fragrant as the tomatoes soften.
Pour in the rinsed red lentils and coconut milk, then add 2 cups of water. Bring the mixture to a gentle boil, with bubbles just beginning to form around the edges.
Reduce the heat to low, cover the pan, and let it simmer gently for 20 to 25 minutes. Stir occasionally, watching as the lentils soften and the sauce thickens into a velvety, vibrant orange mixture.
Remove the lid, taste the curry, and season with salt as needed. Squeeze in the lemon juice to brighten the flavors, then cook for another minute.
Serve the hot, creamy lentil curry over fluffy rice or with warm bread, enjoying the rich aroma and smooth, hearty texture.