Heat 1 tbsp oil in a large non-stick frying pan over a medium-high heat. Add the capers and fry until puffed and crisp, about 2 mins. Transfer with a slotted spoon to a plate covered with kitchen paper.
Zest the orange and set aside. Cut it in half and place, cut-side down, in the hot oil. Caramelise for 3-5 mins, then set aside. Wipe the pan out with kitchen paper. Put the flour on a plate and season. Dust the fish fillets lightly in the seasoned flour. Heat the remaining oil over a high heat. Add the fillets, turn the heat down slightly, then cook for 1-2 mins each side. Transfer to a plate and keep warm.
Add the butter to the pan, along with the blood orange zest. Once melted, squeeze the juice of one of the orange halves into the pan, along with a squeeze of lemon juice, the chilli flakes and thyme. Whisk to combine, taste, then add more orange juice if you think it needs it. Add the capers back to the pan, followed by the fish fillets, basting them briefly in the butter. Divide between two plates and spoon over any excess butter. Serve with wilted spinach.