This is no ordinary lemon sponge. Combining lemon juice with thyme gives the bundt a heady, botanical quality. Don’t hold back on the thyme – the recipe calls for a lot, and (because of it) the results are out of this world.
Make the sponge. Heat the oven to 180°C/160°C fan/350°F/Gas 4. Generously grease the bundt tin with melted butter, making sure you brush it between the grooves to prevent the sponge sticking when you turn it out.
Beat the butter and sugar in a stand mixer fitted with a beater, on medium speed for 3-5 minutes, scraping down the inside of the bowl from time to time, until the mixture is pale and creamy. One at time, add the eggs, beating well between each addition, and adding a spoonful of the flour if the mixture starts to curdle.
Sift the flour and baking powder into the bowl and mix on a low speed until combined.
Add the lemon zest and thyme leaves and mix until combined. Stir in the lemon juice to give a thick dropping consistency.
Spoon the mixture into the prepared bundt tin and, using a spatula, gently push the batter into the grooves and up the sides of the tin, then smooth the top. Tap the tin on the work surface to release as many air bubbles as possible. Bake the sponge for 35-40 minutes, until risen and golden, and a skewer inserted into the centre comes out clean.
Make the lemon-thyme syrup. While the sponge is baking, pour the lemon juice into a small pan, add the sugar and bring to the boil. Add the thyme, turn the heat down slightly and simmer for 10 minutes, until the liquid has reduced to a syrup consistency. Pass the syrup through a sieve into a bowl and leave it to cool. Discard the thyme.
Make the crystallised lemon. Using a potato peeler, peel the lemon in strips, running from top to bottom, then cut the lemon peel into matchsticks. Place the peel in a small pan, cover with water and bring the water to the boil. Immediately remove the pan from the heat and pass the water through a sieve. Tip the lemon matchsticks back into the pan and repeat the boiling and draining process once more to get rid of any bitterness in the peel.
Place the drained lemon matchsticks in a pan with the 100g of caster sugar and 100ml of water and bring to the boil. Turn the heat down slightly and simmer for 15 minutes, until the peel strips are translucent and soft. Pass them through a sieve, then spread the strips out on the lined baking sheet and sprinkle them with sugar. Toss to coat each piece of lemon peel in the sugar.
Make the crystallised thyme. Pour the egg white into a shallow dish and lay the thyme sprigs in it. One by one, remove the sprigs and wipe them almost dry with kitchen paper. Place them next to the lemon peel and sprinkle with sugar. Leave the sheet in a warm place for the thyme and lemon peel to crystallise and dry out.
Meanwhile, remove the bundt from the oven and leave it to cool in the tin for 5 minutes, then turn it out onto a wire rack.
While the sponge is still warm, use a thin skewer to prick holes over the surface and brush with the lemon-thyme syrup. Be patient and allow the syrup to sink into the sponge before adding more, then leave the bundt to cool completely.
Make the icing. Sift the icing sugar into a bowl and add enough lemon juice to mix to a stiff, but pourable icing (you may not need all the juice). Spoon the icing over the bundt cake, leaving it to drip down the ridges, and then decorate with the crystallised lemon and thyme.