In a 6-quart slow cooker, combine the butter and rice; stir until the butter is melted and the rice is completely coated. Stir in the bananas, brown sugar, allspice, and salt. Stir in the almond milk and vanilla extract. Cover and cook on low for 2½ to 3 hours. Serve immediately or switch cooker to the warm setting until ready to eat. Just before serving, stir ½ cup half and half into the risotto. Ladle risotto into bowls and drizzle remaining half and half over top. Sprinkle with chopped walnuts.