Combine blueberries, half the sugar, half the lemon juice, and the lemon zest in a saucepan. Bring to simmer over medium heat, mashing berries slightly as you stir until berries burst

Transfer the mixture to a food processor and puree until smooth.

Measure out a 1/2 cup of the puree and place it back into the saucepan. Reserve the remaining puree for the blueberry lemon mousse.

Bloom the gelatin by cutting the sheets into 1-inch strips and submerging them in cool water for 5-minutes to soften.

Add remaining sugar and lemon juice to the saucepan with the puree. Heat until it begins to bubble.

Wring out gelatin to remove excess water and add it to the warm blueberry sauce. Whisk to dissolve.

Pour the mixture into an 8x8 inch pan lined with parchment paper. Refrigerate for at least 4-hours until set.

When you are ready to assembler the cake, you will remove the gelée from the fridge and cut a 5-inch round out of the center.