Heat your oven to 220°C/Fan 200°C/Gas 7. Put the pork fillet in a roasting tin and season with salt and pepper. Peel and finely grate the ginger. Crush 2 tsp coriander seeds in a mortar and pestle, with a spice grinder or use a bowl and the bottom of a jar.
Mix the spices together with ½ tbsp olive oil and rub over the fillet. Roast for 15 mins.
Tip the rice into a small pan. Pour in 150ml boiling water. Cover. Bring to the boil. Turn the heat right down. Simmer for 20-25 mins till all the water is absorbed. Take off the heat. Set aside, lid on, for 2-4 mins to finish cooking the rice.
While the pork and rice cook, zest the orange, then chop half of it into wedges. Chop the rhubarb into 3 cm chunks and toss in a bowl with the honey and orange zest. Add the honeyed rhubarb to the pork roasting tin.
Turn the oven down to 200°C/Fan 180°C/Gas 6. Slide the roasting tin back into the oven. Cook for 10-15 mins more till the rhubarb is tender and the pork is just cooked – you want it a little pink and juicy in the middle.
Tip the watercress into a bowl and dress with a pinch of salt and pepper, a squeeze of juice from the remaining orange half and 1 tsp olive oil.
Roughly chop the coriander. Slice the pork and serve it with the rhubarb, rice and watercress. Scatter over the chopped coriander and serve with the orange wedges for squeezing.