Discard mushroom stems, and slice caps into 1/4-inch-thick slices.
Melt butter in a large saucepan coated with cooking spray over medium-high heat. Add mushrooms, onion, and garlic; sauté 2 minutes. Stir in orzo and broth. Bring to a boil; reduce heat, and simmer, uncovered, 13 to 15 minutes or until orzo is tender and liquid is almost absorbed, stirring frequently. Gently stir in pepper and sage. Serve immediately.