Cut the mushrooms into slices 5mm thick. Arrange the mushrooms on two baking trays in a single layer without overlapping.
Mix the oil, garlic powder and herbs together in a teacup, then brush both sides of the mushrooms with the oil mix using a pastry brush.
Bake in the oven for 30 minutes, then turn each piece over and arrange them in a single layer again. Return to the oven for another 10 minutes, until the mushrooms are golden brown and crispy. Sprinkle with a little salt to taste.