Heat olive oil and butter in a skillet and add garlic. Saute the garlic till fragrant and add the mushrooms. Toss the mushrooms once and then cook them on medium high heat, tossing them again a few minutes later. Cook for a total for 4-5 minutes or till the mushrooms are tender.
Add balsamic vinegar, soy sauce, sugar, pepper and rosemary and toss to coat the mushrooms in the sauce. Cook for another minute or two and serve topped with chopped green onions or parsley.