Gather all the ingredients.

Remove stems from the mushrooms.

Wipe the cap of the mushroom with a damp paper towel.

Scrape the black gills out of the cap with a spoon.

Slice the mushrooms into ¼ inch strips.

Oil in a large skillet over medium heat.

Add shallots and cook until translucent.

Add mushrooms and occasionally stir, and cook until wilted.

NOTE: Work on batches to easily stir while cooking.

Add bell pepper, garlic, salt, pepper and rosemary.

Stir and cook until all the liquid has evaporated.

Stir in vinegar and toss once more. Serve warm.