1. Preheat oven to 350 degrees. Butter six 1 1/2- to 1 3/4-cup oval gratin dishes; set aside.

2. Bring milk, butter, 1/2 cup sugar, the zest, and salt to a simmer in a medium saucepan. Add bread; remove from heat. Let stand, stirring often, 30 minutes.

3. Stir in yolks. Divide evenly among prepared dishes. Bake until just set, about 15 minutes. Let cool on a wire rack 30 minutes. Reduce temperature to 325 degrees.

4. Heat jam in a small saucepan until thin. Spread over tops of puddings.

5. Put egg whites and remaining 1/2 cup sugar into the heatproof bowl of an electric mixer set over a pan of simmering water. Whisk until sugar has dissolved and whites are warm, about 2 minutes. Attach bowl to mixer fitted with whisk attachment. Beat on high speed until medium peaks form. Mound meringue over puddings, leaving a 1/2-inch border.

6. Bake until meringue is golden all over and puddings are heated through, about 10 minutes. Serve immediately.