Preheat oven to 300°F.
Slice mushrooms thinly, no more than 1/4 inch thick.
Brush both sides with olive oil; spread on parchment paper lined baking sheets. (You’ll need 2.)
Season with salt and pepper.
Place one tray in the top half of the oven and the other in the bottom half; bake for 30 minutes.
Switch trays to top and bottom; bake for another 30 minutes.
Check for crispness; a few additional minutes may be needed.
Portobellos will need additional time, approximately 10 minutes.