Melt butter in a large skillet over medium heat. Add onions and saute until soft, about 2-3 minutes. Add garlic and flour and cook, stirring constantly for 2 minutes. While whisking, slowly pour in evaporated milk. Whisk well until smooth. Cook mixture, stirring constantly until it begins to thicken then pour into a slow cooker along with chicken broth, diced broccoli, salt, pepper and thyme. Cover with lid and cook on high heat for 2 1/2 - 3 hours or low heat for 6 hours.