Wash, stone and cut the plums. Bring the lemon zest, sugar and about three quarters of the juice to the boil. Mix the remaining juice with the cornstarch and mix with the fruit. Let simmer for about 3 minutes.
Cover the groats and cool for at least three hours.
Remove the lemon peel.
Whip the cream until thick. Mix in the honey and vanilla. Serve with the grits.