To caramelize sugar, in a large skillet, cook the 2/3 cup sugar over medium-high heat until sugar starts to melt, shaking skillet occasionally. Do not stir. When sugar starts to melt, reduce heat to low and cook about 5 minutes or until all of the sugar is melted, stirring as needed with a wooden spoon. Pour caramelized sugar into the bottom of a 9-inch deep-dish pie plate; tilt pie plate to coat bottom. Let stand for 10 minutes.
Meanwhile, preheat oven to 325°F. In a large bowl, beat cream cheese with an electric mixer on medium to high speed for 30 seconds. Add the 1/2 cup sugar; beat until combined. Beat in sweetened condensed milk until combined. Beat in eggs, half-and-half, and vanilla until smooth.
Place pie plate in a roasting pan on an oven rack. Pour egg mixture into pie plate. Pour boiling water into the roasting pan around pie plate to the depth of 1 inch. Bake about 70 minutes or until top is lightly browned and a knife inserted near center comes out clean.
Remove pie plate from water. Cool on a wire rack. Cover and chill for 8 to 48 hours.
To unmold, loosen edge of flan with a knife. Invert a serving plate with a rim over pie plate; turn plate and dish over together. Remove pie plate. Spoon any caramelized sugar remaining in the pie plate onto flan. If desired, garnish with Sautéed Plantain Slices and nuts.
Peel and slice 1 small plantain. In a small skillet, heat 1 tablespoon butter over medium-high heat. Add plantain slices; cook about 8 minutes or until lightly browned, stirring occasionally. Cool.