From America’s Test Kitchen
"Though a mixture of berries offers a wonderful combination of color, flavor, and texture, it’s also fine to use just one or two types of berries. One-half pint of fresh berries equals about one cup of fruit. Later in the season, ripe, peeled peach or nectarine slices can be used in combination with blueberries or raspberries. We recommend using only fresh fruit, but if you must use frozen, raspberries are the best option. Do not thaw them before baking. Avoid using a metal pie pan that may react with the acidity of the fruit and impart an “off” metallic flavor. Serve the fruit gratin with lightly sweetened whipped cream or vanilla ice cream."