Season the mullet inside and out and stuff some of the thyme sprigs inside the fish. Put the fish in an ovenproof dish where they can fit in a single layer.
Mix the olive oil with the lemon juice and the juice of 1/2 the orange and pour this all over the fish. Cut the rest of the orange into thin slices, halve these and tuck them around the fish with the remaining thyme.
Season and drizzle over a little more olive oil. Bake in an oven preheated to 180 C, 160 Fan, Gas 4 for 15 minutes. Add the olives and return to the oven for a further 10 minutes. Check that the fish is cooked through and serve immediately.