Heat a medium pan over medium heat. Add the oil, and heat until warm. Add the mushrooms and onions. Cook for 2 to 3 minutes until softened. Add the garlic, salt, and pepper. Saute for an additional minute then remove from heat.
1 tablespoon olive oil, 1 pound baby bella mushrooms, ¼ cup white onion, 1 garlic clove, ¼ teaspoon salt, ⅛ teaspoon ground black pepper