Place mushrooms, garlic, basil, oregano, thyme and bay leaves into a 3-qt slow cooker. Stir in vegetable broth; season with salt and pepper, to taste.
Cover and cook on low heat for 3-4 hours or high heat for 1-2 hours, or until browned and tender. Stir in half and half and butter during the last 15-20 minutes of cooking time.
Serve immediately, garnished with parsley, if desired.
Notes
*Half and half is equal parts of whole milk and cream. For 1 cup half and half, you can substitute 3/4 cup whole milk + 1/4 cup heavy cream or 2/3 cup skim or low-fat milk + 1/3 cup heavy cream.