Soak the gelatin sheets in a bowl with cold water for 5 to 10 minutes.
In the meantime, using a sauce pan, simmer the cream, milk, feta cheese, vanilla seeds and sugar until dissolved. Remove from heat.
Strain the gelatin sheets, add them to the saucepan and stir. Pour the mixture into panna cotta molds and refrigerate for at least 4 hours until firm.
For the cherry tomato sauce
Place the cherry tomatoes in boiling water for 1 minute, quickly remove and place in an ice water bath. Peel the skins from the tomatoes and set aside.
Mix brown sugar, water, honey, mastic powder and lemon juice in a saucepan. Simmer on low heat until the syrup thickens. Add the peeled cherry tomatoes and cook on low heat for 40 minutes.
Remove the panna cotta from the molds, top with cherry tomato sauce and serve.