Preheat the oven to 160°C fan. Butter a 23cm tin and line the base with baking parchment.
Place the rhubarb with the caster sugar and orange juice in a small baking tin cover with foil and bake for 15-20 minutes or until just tender. Pour the juices in a small saucepan, then add the orange blossom water & thyme. Heat and reduce until syrupy. Add more orange blossom water to taste if needed. Set aside.
Sift the flour, baking powder and salt, into a large bowl. In another large bowl, using a hand-held electric whisk, or in a food mixer, beat the sugar, cooled melted butter, eggs and vanilla until thick and creamy; this will take a few minutes on high speed. Once it's pale and creamy, with a large spoon, fold in the flour mixture, alternating with the milk. Don't over-mix.
Spoon the batter into the prepared tin, then place the rhubarb on the surface, pushing some of it down into the cake batter, bake for 35-40 minutes, or until a skewer comes out clean.
Cool the cake in the tin for about 10 minutes, then remove and brush with the orange syrup. Serve warm from the oven, with a bowl of crème fraîche or Greek yogurt. (Can be gently reheated if you want to serve it as a dessert).