Heat a tablespoon of oil in a heavy-bottomed pot over medium heat until it shimmers and starts to gently ripple.
Add the chopped onion and cook, stirring often, until it becomes translucent and soft, about 5 minutes. The onion should smell sweet and earthy as it softens.
Toss in the minced garlic and grated ginger, stirring constantly for about 30 seconds until fragrant, filling the air with a warm, spicy aroma.
Stir in the turmeric and cumin, allowing the spices to toast for about 1 minute until they release their aroma and turn a slightly darker shade.
Add the rinsed red lentils to the pot, stirring to coat them in the fragrant spice mixture, then pour in water or broth.
Bring the mixture to a boil over high heat, then reduce the heat to low and let it simmer gently, uncovered or slightly ajar, for about 20-25 minutes until the lentils are tender and the curry thickens.
Stir occasionally to prevent sticking and check the texture; the lentils should be soft and breaking down slightly, creating a creamy consistency.
Once cooked, taste and adjust the seasoning with salt; then squeeze in fresh lemon juice for brightness, stirring well to combine.
Let the curry rest for a few minutes off the heat, allowing the flavors to meld and the texture to settle.
Serve the warm, thick lentil curry with rice, bread, or your favorite side, garnished with fresh herbs if desired.