In a medium pot, heat butter over medium heat until foamy. Add shallots and garlic, cook until soft, about 5 minutes, stirring occasionally. Add chopped broccoli, 3 cups chicken broth and oregano. Bring to a simmer, cover and cook until very tender, 15-20 minutes. Stir in potato flakes and Dijon mustard. Allow to cool for 5 minutes, stirring occasionally.