Your mixture will be a little soupy after adding the broth. The flour will help to thicken it, and you do want to have a little bit of liquid in the meat mixture when you top it with potatoes so that the bottom doesn't burn during the last part of cooking. If your mixture is very liquidy, cook the mixture until some of the liquid evaporates and you can see a thin layer of broth on the bottom of the Dutch oven. If your mixture is dry, add extra broth or water to ensure that there is a thin layer of liquid in the pot before topping with the mashed potatoes.