For the grapefruit syrup heat together the fresh grapefruit juice, 30gr sugar and the hibiscus flowers until slightly thick, syrupy and tinged dark red. Let cool.
For the yogurt base dissolve the cornstarch in 50ml milk, let sit for a while.
In a small saucepan combine the leftover whole milk, heavy cream, 85gr sugar, pinch of salt, the agave syrup and zest of 1 grapefruit, stir together, place on medium heat, bring to a boil, take off heat, cover and let steep for 10min. Sieve the mixture, return into the saucepan, bring to an almost boil, add the cornstarch milk and cook on medium constantly stirring until thickened. Take off heat, add 300gr Greek yogurt and the grapefruit syrup, mix until combined. Cool to room temperature then transfer into the fridge overnight.
Freeze in your ice cream maker according to manufacturer's instructions adding crushed meringues at the last couple of minutes of churning. Pair with pink and white grapefruit sorbets.