Start the oven heating to 200 C / 400 F / Gas Mark 6.
Cut the parsnips into lengthwise wedges, removing the tough centre cores if there are any. Peel and slice the rest of the root veg. Place all the root veg in a large roasting tin, toss in 3 tablespoons of the olive oil to coat them all, season with salt and pepper, and roast for 20 minutes.
While this is happening, heat the remaining tablespoon of oil in a frying pan, and cook in it the herbs, mushrooms and garlic until the garlic is tender.
Remove the veg from the oven, mix the mustard in to coat, and return to oven until the veg are tender and caramelized, about 15 to 20 minutes.
While this is happening, mix the eggs, flour and milk together to make a batter.
Remove the veg from the oven when carmelized and tender, arrange in a casserole dish, sprinkle the mushroom mix over the veg, then pour the batter over it all.
Cook for 20 minutes, then remove and place the cheese slices over top, then bake in oven for a further 15 to 20 minutes or until golden.
If you want, you can use 1 turnip, 1 swede, 1 rutabaga, etc, whatever you have available. You could also try other root vegetables such as potatoes, Jerusalem artichokes, etc., to the appropriate quantity.