Preheat oven to 375℉
In a large skillet, melt the vegan butter
Add sliced onion and garlic, sauteeing until the onion is soft and translucent, about 5 min.
Stir in the flour, and cook for another minute
Gradually whisk in the milk and broth. Bring mixture to a simmer, stirring constantly until it thickens slightly
Add the thyme, salt and black pepper, then mix in nutritional yeast.
Layer half of the sliced turnips in a greased baking dish. Spread half of the mustard greens over the turnips.
Pour half of the sauce over the greens, then repeat layers with the remaining turnips, mustard greens, and sauce.
Sprinkle breadcrumbs evenly over the to for a crispy crust
Cover the dish with foil and bake for 25 minutes. Uncover and bake for another 10-15 minutes, or util the turnips are tender and the top is golden brown