Step 1Combine the cream, milk and sugar in a saucepan and cook over medium heat for 5 minutes.
Step 2Combine one-half cup pistachios and the lemon zest in a food processor. Pour the heated cream mixture over the top and pulse 3 or 4 times to break up the pistachios. Transfer to a covered container and refrigerate 2 hours to infuse the flavors.
Step 3Place the gelatin in a large stainless steel mixing bowl. Pour the water over the gelatin, adding more water if necessary to moisten all of the gelatin.
Step 4Place the mixing bowl over a saucepan of barely simmering water and heat until the gelatin melts, about 30 seconds. Do not stir the gelatin, as it will scald on the sides of the mixing bowl. Turn off the heat and keep the gelatin warm.
Step 5Using your fingers, lightly coat 4 (one-half-cup) ramekins with oil.
Step 6Bring the cream mixture to a simmer and pour it through a fine mesh strainer into the gelatin. Whisk to combine thoroughly, scraping the bottom of the bowl to free any gelatin that may have solidified there.
Step 7Divide the mixture evenly among the prepared ramekins and refrigerate for 6 to 8 hours. To unmold the panna cotta, run a thin knife around the side of each ramekin, dip the bottom in hot water for a few seconds and invert onto a plate. Sprinkle with the remaining 1 tablespoon chopped pistachios.