Heat a skillet over medium heat. Add in the olive oil and sauté the onions until they are translucent, about 3-4 minutes.
Add the garlic and sliced mushrooms, and cook until the mushrooms are tender and have released their moisture and shrink, about 5-7 minutes.
Transfer the sautéed onions, garlic, and mushrooms to the slow cooker.
Add the Arborio rice, vegetable broth, white wine, dried thyme, and dried parsley. Stir well to combine.
Cover and cook on high for 2-2.5 hours or until the rice is tender and has absorbed most of the liquid. Stir occasionally to ensure even cooking and no sticking.
Once the rice is cooked, stir in the Parmesan cheese, heavy cream, and butter. Mix until well combined and creamy. Test the risotto and add in salt and pepper as needed.
Serve the risotto hot, garnished with fresh parsley and a drizzle of olive oil if desired.