I've never used portobello mushrooms before but found some on sale, so pulled up America's Test Kitchen for tips. Wash, remove the woody stem and gills. They did a demo cooking other mushrooms that I did with these; since I won't be using them as a slab the way they usually are I did a large dice then put them into a dry hot non-stick skillet until they gave up a lot of their moisture. THEN you add a little oil (it seems mushrooms are thirsty and that's why the oil disappears if you add it first). They smell wonderful and are now cooling on a tray in the fridge. I'll use them in the next batch of marinara sauce.