Preheat your oven to 400F.

Place flour and spread evenly in a large diameter cast iron skillet or other oven-safe, heavy pan

Place in the center of the preheated oven.

Bake for 60 to 70 minutes stirring every 15 minutes at first (I use a wisk or flat spatula).

After color develops on the flour, stir every 10 minutes or more often if near the end of the cook.

For a dark roux, the flour will be a dar tan. To judge color as you would with an oil-based roux, place a very small amount of browned flour in a few drops of water and press with the back of a spoon. The sample will be the color of the roux when added to a dish and for the time given will be a mahogony color.