Heat olive oil in a large skillet over medium heat. Add onion and garlic; cook until soft, about 5 minutes. Stir in mushrooms and pepper flakes; cook until mushrooms are coated with oil and begin to soften.
STEP 2
Add sherry and chicken broth; simmer until liquids are reduced by half. Add arugula; cook until wilted, about 1 minute. Season with black pepper. Serve immediately.