I have long made oven-roasted red-skin potatoes, sweet potatoes, and carrots in much this same way, but recently discovered that I can almost *ALMOST* fool myself into thinking I am eating those yummy potatoes with these turnips...
In a high-sided 13x9 in pan (I use an old metal one because I find it gives me the best caramelization on the turnips/onions), combine turnips, onions, and garlic. Spray generously with cooking spray and toss. You could also use olive oil.
Add all seasonings. I usually start with about 2 tsp of salt and 1/2 tsp of black pepper and then taste them each time I stir and adjust those seasonings to taste, but I am a salt-fiend : }.
Bake for 15 minutes, then remove from oven and stir to allow the veggies to cook evenly. (I taste for seasoning at this point.)
Return to oven and bake another 15 minutes. Remove and stir again. Depending on how you like your turnips and how thick you sliced them, they may be done at this point. I like mine pretty well done and with some nice caramelization so I return them to the oven and bake an additional 10-15 minutes.