Add all the ingredients except the agar to a medium saucepan and bring to the boil. Stir the mixture a few times as it heats with a rubber spatula to stop the bottom scorching. Remove from the heat and leave to infuse for 30 minutes.

Blend as finely as possible in a liquidiser or using an immersion blender. Strain through a fine-mesh sieve, back into the saucepan (make sure there are no residual fibres in the pan). Add the agar and bring to the boil again (this activates the agar), stirring with a spatula as it reaches the boil. Make sure the mixture really bubbles for at least 30 seconds before removing from the heat.

Pour the mixture into a container, cool to room temperature and place in the fridge to set. This cooling process takes 2-3 hours. When it is fully set, blend the custard to a smooth cream.

Cut the bread or baguette very thinly. A thin baguette is easiest for making complete canapé rounds, otherwise cut the larger slices of bread into bite-sized pieces. Toast for a few minutes in a hot oven (about 200c). If you put the bread pieces directly on the oven shelf, you will not need to turn them.

When cool, shave small pieces of 'Parmesan' with a vegetable peeler (omit for vegan) and lay over the toasts.

Slice the radishes very thinly - this is easiest with a mandoline - and add to the Parmesan, if using.

Top with a blob of asparagus custard. This is easiest with a piping bag, but a small spoon will also do.

Finish with a tiny sprig or leaf of fresh herb.