Trim stems and clean mushrooms with a damp paper towel on the outside to remove any dirt. Place them in a 6 quart crockpot.
In a small bowl, melt the butter and add garlic. Add the chopped chives, parsley, dill, onion powder, garlic powder, salt and pepper. Whisk well to combine. Add in the vegetable broth and whisk. Pour over the mushrooms and stir to coat.
Set the slow cooker to low and cook for 3 hours for whole and 1.5-2 hours for sliced. About 15 minutes before the end time, add in the heavy cream (optional) and give the mushrooms a stir.
Garnish with fresh chopped parsley and serve. Store any leftovers in an airtight container in the refrigerator for up to 2 days.