Season chicken with oil, salt, pepper, 1/2 the herbs. Roast in the oven until an internal temp of 165 F, about 30-40 minutes.
Heat a saute pan with a little oil and add in the onions, parsnip, turnip cauliflower, herbs, salt and pepper. Cook 2-4 minutes. Add in the flour and stir together. Stir in the milk and bring to a boil. Simmer and reduce to a thicker sauce consistency. Stir in the cheese and adjust seasoning.
Pour into a baking dish and bake for 15-30 mins
Let the cauliflower bake sit for 5 minutes to rest before serving