Heat oil in a pot over medium-low heat. Add onions, garlic, and ginger; cook and stir until fragrant, 3 to 5 minutes. Sprinkle in sugar and cumin; cook and stir until sugar caramelizes, about 2 minutes, making sure cumin does not burn.

Stir in curry paste and cook for 1 to 2 minutes. Add potatoes, tomatoes with their juices, coconut milk, lentils, and bouillon; bring to a boil. Reduce the heat, then cover and simmer, stirring occasionally, until potatoes and lentils are cooked through, 20 to 25 minutes. Season with lemon juice, salt, and pepper.