Warm olive oil in a large pot or saucepan over medium heat. Add the chopped onion and cook until softened (about 5‑6 minutes).

Stir in the garlic, ginger, Thai red curry paste, and red pepper flakes. Cook for 1‑2 minutes or until fragrant, stirring so the paste doesn’t burn.

Add the red lentils and stir to coat them with the onion‑spice mixture.

Pour in the crushed tomatoes, add the fish sauce (or soy sauce), and the 3 cups of water. Let the mixture come to a boil, then lower the heat to maintain a gentle simmer. Cover partially and simmer until the lentils are cooked through — about 20 minutes.

After the lentils are tender, stir in the full‑fat coconut milk, lime juice, brown sugar, salt and black pepper. Continue cooking a few more minutes until everything is heated through and the flavours meld.

Ladle the soup into four bowls. Garnish with a swirl of extra coconut milk, fresh cilantro, and lime wedges for a bright, fresh finish.