Place the sliced mushrooms, diced onion, minced garlic, vegetable broth, thyme leaves, bay leaf, salt, and black pepper into the slow cooker.
Stir gently to combine all the ingredients evenly.
Cover and cook on low for 6 to 7 hours until the mushrooms are soft.
Remove the bay leaf, then stir in the heavy cream until fully incorporated.
Taste and adjust salt or pepper if needed before serving.