What is the French name given to a type of long, slender green bean that has an intense flavor?

The French name for the long, slender green bean known for its intense flavor is "haricots verts." This term translates directly to "green beans" in English, but specifically refers to the younger, more tender varieties that are typically thinner than standard green beans. Haricots verts are often prized in French cuisine for their vibrant color, crisp texture, and more concentrated flavor, making them a popular choice in various dishes and salads.

Fava beans, string beans, and butter beans are distinct types of legumes or beans, each with their own characteristics and culinary uses, but they do not refer specifically to the narrow French green beans that's being asked in the question. Hence, haricots verts is the correct and precise designation for that specific type of green bean.