Heat 2 tablespoons butter in a large saucepan over medium heat until melted.
Add the finely chopped small onion and 1 minced garlic clove; sauté, stirring occasionally, until the onion is translucent, about 5 minutes.
Pour in 1 2/3 cups vegetable broth and bring the mixture to a boil.
Add 1 pound frozen lima beans, reduce heat to low, cover, and simmer until the beans are tender, about 30 minutes.
Season the beans with salt and black pepper to taste, and add red pepper flakes if using.
Serve topped with freshly grated Parmesan and chopped parsley.