Make the vegetable purées by putting the spinach, carrots and beetroot into three separate saucepans. Add just enough boiling water to cover the vegetables then bring each pan to a simmer. The spinach will only take a few minutes to wilt. Once softened, spoon out the spinach leaves into a stick blender pot or a jug. Add in 150 ml of the cooking water and use the stick blender to blitz to a smooth puree.
Cook the carrot and beetroot until soft, then repeat the process - using 150ml of the cooking water each time.
To make the dough, divide the flour between 3 mixing bowls. Add 1 teaspoon of salt to each, and a third of the yeast - so just over 3g to each bowl. Working with one bowl of flour at a time, add in 160ml of the luke-warm puree and mix it all together. It should be a soft dough but not sticky. Add 10ml more purée if you need to. Knead on a floured worktop for 10 minutes, or in a stand mixer for 5. Put it into a clean mixing bowl and cover with cling film. Repeat with the other two doughs.
Prove the doughs for 90 minutes, or until doubled in size. Knock each dough back then separate into portions - a digital scale will be handy here. Divide each colour dough into 5 balls weighing 55g, and 4 balls weighing 45g. You will have a small amount left over from each dough.
To create the wreath, take the Camembert out of its wrapper but keep the bottom of the wooden tray. Put this into the middle of the baking tray with the Camembert inside it. Position 10 of the smaller balls around the edge of the Camembert, in alternate colours, evenly spaced and not quite touching each other. Arrange the larger 15 balls in an outer circle, again, alternating colours and not quite touching the other dough balls. Cover the dough loosely with clingfilm and leave to rise for another 30 minutes until the dough has risen again by at least half.
Preheat the oven to 180°C / 160°C fan while the dough is rising. Once the dough has risen, remove the cling film and make small cuts in the top of the Camembert rind. Insert thin slices of garlic into the cuts, and a few sprigs of rosemary. Drizzle the olive oil on top.
Lightly egg wash the dough balls, then bake for 25-30 minutes until the dough has risen and is cooked through. Serve warm.