Set a large pot or Dutch oven on the stove over medium heat.

Add the olive oil to the pot and warm it for about 30 seconds.

Add the onion, carrots, celery, and 1 teaspoon of the salt to the pot.

Cook the vegetables for 6 to 8 minutes, stirring every so often, until they look softened and the onion is no longer crisp.

Add the garlic and apple to the pot.

Cook for 2 minutes, stirring often, until the garlic smells strong but does not brown and the apple starts to soften at the edges.

Sprinkle the curry powder and black pepper into the pot.

Stir for 30 seconds so the spices coat the vegetables and smell toasty.

Add the red lentils to the pot and stir once so they are mixed through the vegetables.

Pour the vegetable broth into the pot and scrape the bottom with a spoon to loosen anything stuck there.

Turn the heat to high and bring the soup to a full boil.

Reduce the heat to medium-low so the soup stays at a gentle simmer, with small bubbles around the edges of the pot.