Hello Emma. Watery mince usually happens when the meat and vegetables release their natural juices during cooking and the liquid is not reduced enough before assembling the pie. Adding too much broth or stock without boiling it down will also cause a thin gravy.
To fix this and get that stodgy texture you want, you can make a cornstarch slurry by mixing cornstarch with a little cold water, then stirring it into the simmering meat base until it thickens up. Alternatively, you can mash a tablespoon of butter and flour together into a paste and stir that in to simmer for a few minutes. This will safely support your mashed potato topping without it sinking.
