Preheat oven 350°.
Spray or butter soufflé dish (5 inches deep and 10 inches in diameter).
Toss turnip, fennel, and leeks in a bowl with olive oil, salt and pepper.
Roast for 15 minutes.
In the meantime mix together eggs, milk, and yogurt add salt, pepper and nutmeg.
Add a single layer of bread cubes in the bottom of soufflé dish.
Add a layer of root vegetables and then gruyere.
Repeat until all ingredients are used.
Press into dish with hands.
Slowly pour egg mixture into dish until all is absorbed.
To help the egg mixture absorb better, poke holes through the top of the dish.
Refrigerate for at least 3 hours or overnight.
Bake for 40 minutes.
Serve warm.