In a large pot, melt butter over medium heat. Add the chopped onions and cook until just beginning to soften, 3-4 minutes. Add the garlic and potatoes. Cook until just fragrant, 30 seconds to 1 minutes. Pour in the vegetable broth and bring to a boil. Season with salt and pepper, reduce heat to medium low, cover, and simmer until potatoes are tender, about 15 minutes.