Begin by cleaning the baby bella mushrooms with a damp cloth. Remove any dirt or debris, and slice them into quarters. Mince the garlic and onion, and set everything aside.
In a large skillet, heat the 1 tablespoon butter and 1 tablespoon olive oil over medium heat. Once the butter has melted, add the minced onions. Cook for about 3-4 minutes, or until they become translucent. Then, add the 2 cloves of minced garlic and stir for an additional minute until fragrant.
Add the 1 pound of baby bella mushrooms to the skillet. Sauté for 5-7 minutes, stirring occasionally, until the mushrooms are golden brown and tender.
Pour in the 1 cup of balsamic vinegar and add the 2 teaspoons of soy sauce along with the 1 teaspoon of brown sugar. Stir to combine, allowing the mixture to simmer for about 5 minutes. This will help thicken the sauce and deepen the flavors.
Once the sauce has thickened, remove the skillet from heat. Stir in the 1 teaspoon of minced parsley and 1 teaspoon of minced rosemary for fresh flavor. Taste and adjust seasoning if necessary. Serve hot, and enjoy your Garlic Balsamic Mushrooms as a side dish or topping.