Place peeled and cubed potatoes, sliced carrots and diced onion into a large crock-pot. Add in frozen peas, broth and seasonings.
Cover and cook on low for 6 hours or on high for 3-4 hours. In a medium saucepan over medium heat, melt butter. Add flour and let cook for a minute to brown. Add in milk and let cook over medium heat, while whisking constantly until sauce has thickened.
Pour the sauce into the crock-pot, cover and let cook for another hour.