Preheat oven to 350℉. Grease a 9-inch cake pan or cast-iron pan with butter, and sprinkle a fine layer of flour over top.
For the Compote:
In a small saucepan, over medium heat, add rhubarb, sugar, and water. Cook, stirring occasionally, until rhubarb has fully softened. Strain and reserve juice. Set rhubarb aside to cool while you make the rest of the cake.
For the Cake:
Cream butter and sugar together until smooth. Add eggs, one at a time, mixing well after each addition until batter is light and fluffy, then mix in lemon juice. Gently mix in the baking powder, salt, thyme, and flour, being careful not to over mix.
Pour the batter into the prepared pan and smooth. Spoon the rhubarb compote over the top of the batter then, using a butter knife, swirl the compote through the cake batter, being careful not to scrape the bottom of the pan.
Drizzle 2 tablespoons of the reserved rhubarb compote juice over the top, sprinkle with the lemon zest, thyme, and sugar.
Bake 25 minutes or until knife inserted in center of cake comes out clean.
Serve warm or room temperature. Enjoy with whipped cream or Greek yogurt, served on the side.
Notes: You can substitute fresh berries or sliced stone fruit for the rhubarb compote if rhubarb is not available.
Any leftover juice from the rhubarb compote can be chilled and used as a sweet-tart ingredient in cocktails and mocktails, or enjoy a splash in sparling water.