Line an 8-inch (20 cm) square baking pan with parchment paper. Cut each plum in half, but not all the way through and remove pit. Place plums cut side up in prepared pan (they will be snug). Drizzle with honey. Roast in 425ºF (220ºC) oven until very soft, darker in colour and caramelized on the bottom, 1 to 1-1/2 hours. Let cool 10 minutes. Transfer to food processor and process until smooth. Pour into container, cover and refrigerate overnight.

Custard: In medium bowl, lightly beat egg; set aside. In small saucepan, over medium heat, whisk together cream, milk and honey until honey is dissolved. Cook until mixture begins to steam. Remove 1/2 cup (125 mL) of warm mixture and slowly whisk into  beaten egg. Pour warmed egg mixture back into saucepan, whisking gently. Cook, stirring constantly with wooden spoon, until mixture is thick enough to coat back of metal spoon, about 8 minutes. Pour into heatproof bowl; place plastic wrap directly on surface and refrigerate overnight.

Gently whisk plum purée into custard. Transfer to ice cream machine and follow manufacturer’s directions. Freeze in covered container. 

For the best flavour and texture, remove from freezer 10 to 15 minutes before scooping and serving. Gelato can be stored in freezer for up to 3 weeks.