Since potatoes and turnips are of similar consistency, you can cook them together in the same pot.
Many recipes that call for vegetables aren't exact in their measurements. I could weigh out how much potato or turnip I used, but the reality is that if my turnip is a little over or under, I’m just going to use it anyway. Cooking is an art, and much of what we do is by eye and by taste. Relax in the kitchen and go with the flow.
Pour off the water, add the butter and let the pot sit covered for a few minutes. Using a potato masher, mash until all the potato and turnip is broken down. Add several grindings of black pepper and about half the salt. Pour in most but not all of the milk.
Continue to mash until the mixture is more or less smooth, but with some lumps of turnip. Add more milk if needed to achieve the desired consistency. Taste and adjust the seasonings if required. Run a knife through the thyme leaves or simply rub them through your palms to release their wonderful aroma and stir them into the mash.
For presentation, garnish with a sprig of fresh thyme. (It's amazing how much cred you get for doing something simple like this.)
Potato Turnip Mash https://urbancottagelife.com/potato-turnip-mash/