Heat oil in a large saucepan over medium-high. Add onion; sauté 4 minutes. Add zucchini and garlic; cook 3 minutes. Add tomato, curry powder, and ketchup; cook 1 minute, stirring constantly.
Combine stock and flour in a small bowl. Add stock mixture to pan; bring to a boil. Stir in salt and lentils; cook 3 minutes. Remove pan from heat; stir in lime juice. Divide lentil mixture among 4 bowls; top evenly with yogurt and cilantro.