Curry powders vary in flavor and heat; begin with two tablespoons and add more according to your taste.
French green lentils are small, dark green, and speckled with black; they can be found at some supermarkets and at specialty foods stores.
Heat 1 tablespoon olive oil in heavy large pot over medium heat. Add onion and carrot; sprinkle with salt and pepper.
Cook until onion is translucent, stirring occasionally, about 4 minutes.
Add half of chopped garlic; stir until vegetables are soft but not brown, about 4 minutes longer.
Add 2 tablespoons curry powder; stir until fragrant, about 1 minute.
Add lentils and 4 cups water. Sprinkle with salt and pepper.
Increase heat and bring to boil. Reduce heat to medium; simmer until lentils are tender, about 30 minutes.
Meanwhile, puree chickpeas, lemon juice, 1/4 cup water, remaining 2 tablespoons olive oil, and remaining garlic in processor.
Add chickpea puree and butter to lentil soup. Season to taste with salt, pepper, and additional curry powder, if desired. Add water by 1/4 cupfuls to thin to desired consistency.
DO AHEAD: Soup can be made up to 1 day ahead. Cool, cover, and refrigerate. Rewarm before continuing.
Divide soup among bowls. Sprinkle with thinly sliced green onions and serve with lemon wedges.