Prep the fruit. Rinse the rhubarb and strawberries. Cut rhubarb into even pieces and halve or quarter the strawberries. Pat them dry so you don’t dilute the syrup.

Simmer the filling. Add rhubarb, strawberries, sugar, lemon juice, zest (if using), vanilla, and a pinch of salt to a medium saucepan. Cook over medium heat, stirring gently, for 6–8 minutes. You want the rhubarb just tender and the strawberries juicy but not collapsed into mush.

Taste and adjust. Spoon a bit of syrup and taste. Add a little more sugar or lemon if needed. The syrup should be bright and pleasantly sweet-tart.

Cool slightly. Remove from heat and let the mixture sit for 5 minutes. You want it warm but not boiling hot when it hits the bread.

Prepare the mold. Line your bowl or loaf pan with plastic wrap, leaving plenty of overhang. This makes unmolding easy.

Trim the bread. Remove crusts from the bread. Cut one slice into a neat round (or rectangle if using a loaf pan) to fit the base of the mold. Keep the scraps for patching gaps later.

Coat the bread in syrup. Dip the base piece and a few side slices into the warm fruit syrup for 5–10 seconds each. This stains and softens them without turning soggy.

Line the sides. Press the base piece into the bottom, then line the sides with overlapping slices, syrup-dipped side facing out. Patch any bare spots with trimmed scraps. No gaps means no leaks when you unmold.

Fill with fruit. Spoon in half the fruit and some syrup. Add another layer of bread (not dipped), then the remaining fruit. Finish with a final layer of bread to seal the top.

Weight and chill. Fold over the plastic wrap. Place a small plate on top and weigh it down with cans. Chill at least 8 hours, preferably overnight. The weight compacts the layers and helps the juices soak through evenly.

Unmold. Remove the weights and unwrap the top. Invert a serving plate over the mold, then flip. Gently lift the bowl and peel off the plastic. If it sticks, run a thin knife around the edge and try again.

Serve. Slice with a sharp knife. Spoon any extra syrup over the top. Serve with lightly sweetened whipped cream, a dollop of Greek yogurt, or crème fraîche to balance the sweetness.