Line an 8×8-inch baking dish with parchment paper. Dip the whole lemon in boiling water for about 10 seconds to soften the skin. Trim off the ends, cut the lemon into sections, and chop it finely in a food processor. Add the Meyer lemon juice and process until you reach the desired texture. Strain if you want a very smooth jelly.

Combine the lemon mixture, granulated sugar, and rosemary sprig in a medium saucepan. Bring to a boil and cook for 5 minutes, stirring constantly. Stir in the butter halfway through.

Turn off the heat and remove the rosemary sprig. Stir in the liquid pectin until fully mixed. Continue stirring until the mixture appears even and glossy.

Pour the hot mixture into the prepared pan. Smooth the top if needed and allow it to set at room temperature for about 2 hours until firm to the touch.

Once set, lift the jelly from the pan, place it on a cutting board, and cut into small squares or fun shapes. Roll each piece in granulated sugar before serving.