Cut the potatoes into quarters.
Chop the rutabaga/turnip into chunks.
Chop the carrots and parsnips into thick slices ( about 1" thick).
Put all the root vegetables in a saucepan and add enough water to cover them.
Bring to a boil and cook covered until the vegetables are tender ( about 20 minutes - the rutabaga may need slightly longer).
Remove the carrots and parsnips from the saucepan to a bowl.
Mash them and put them aside to be used in the filling.
Mash the potatoes and rutabaga/turnip together. Set aside for use in the topping.
Chop the mushrooms and heat them with a little water ( 1 Tbsp) in a large pan on Medium-High for a couple of minutes. Cover, reduce the heat and simmer for about 10 minutes, removing the cover for the last minute or two so that they are mostly dry. Transfer to a bowl and set aside.
Reduce heat to Medium and add 1 tbsp water.
Chop the onions, garlic and leek and add to the pan.
Saute, stirring occasionally, for 5-8 minutes until onions start to brown and leeks soften. A little more water may be added, if necessary, to prevent burning.
Crumble the tempeh and mix with the other ingredients in the pan.
Add a little more water ( 1 tbsp) if necessary. Reduce heat to Medium-Low. Saute, stirring frequently for about 15 minutes until the tempeh is starting to get nicely brown.
Sprinkle the thyme (fresh chopped or dried) into the pan and mix, adding a little water ( 1 tbsp) if the mixture is too dry.
Stir the cooked mushrooms and mashed parsnips/carrots into the onion-tempeh mixture.
Pour the filling into a casserole dish and spread it out evenly.
Spread mashed rutabaga potatoes over the onion-tempeh mixture in the casserole dish.
Bake, uncovered, for about 45 minutes until piping hot.
Remove from the oven and let sit, keeping warm, for 15 minutes before serving, to give time for the flavor to develop.