Bake the tart shells as per package directions, and then cool. Meanwhile, heat the oil in a large skillet over medium heat. Add the mushrooms and cook until very tender and all the moisture has been cooked out of them. Add the garlic and chopped rosemary and cook 1 minute more. Add the cream and cook until it slightly thickens; season with salt and pepper and then remove from heat. (If you wish to bake these tarts later, cool mushroom mixture and store in the fridge until ready to stuff, top and bake the tarts.) Remove tarts from their foil liners and set on a parchment-lined baking tray. Fill them with the mushroom mixture and top with small pieces of the goat cheese. Bake in a 350 degree F oven 10 minutes, until well heated through.