Rinse the red lentils thoroughly in cold water until the water runs clear, then set aside.
Heat the oil in a large saucepan over medium heat until it shimmers and starts to smell fragrant.
Add the chopped onion to the hot oil and cook, stirring occasionally, until it becomes translucent and soft, about 5 minutes. You should hear gentle sizzling and see the onion turn a light golden.
Stir in the curry powder and cook for about 30 seconds, releasing a warm, toasty aroma as the spices toast slightly in the oil.
Pour in the vegetable broth or water, bringing it to a gentle boil while you stir to combine the spices evenly into the liquid.
Add the rinsed lentils to the boiling liquid, then reduce the heat to low, cover the pan, and let it simmer gently for about 15-20 minutes. During this time, the lentils will soften, break apart, and create a thick, saucy texture with a beautiful orange color.
Check the curry; if it seems too thick before the lentils are fully soft, stir in a splash more water and continue simmering until the desired consistency is reached.
Once the lentils are tender and have started to break down, squeeze in fresh lemon juice to brighten the flavors. Taste and adjust salt or spice if needed.
Remove the curry from heat, let it rest for a few minutes to allow the flavors to meld, then serve hot over rice or with bread, garnished with herbs if desired.