Preheat oven to 400°F.
In a large skillet, melt butter over medium heat. Add onions and sweat them for 15 minutes or until golden brown. Once the onions are sweet and tender, add the livers and raise the heat just a bit. Cook until the livers are firm and cooked through, another 5-6 minutes. Add salt, pepper and thyme to the livers and onions. Place the mixture in a food processor and grind into a smooth spread. Transfer to a serving bowl.