Cook the pasta in a large saucepan of salted boiling water according to the packet instructions until al dente. Drain thoroughly and return to the pan. Add the butter and toss to coat.
Meanwhile, put the monkfish cubes on a large piece of foil and top with the chillies, garlic and fish sauce. Fold up the edges of the foil and turn them over to seal the parcel. Place on a baking sheet and cook in a preheated oven, 200°C (400°F), Gas Mark 6, for 8–10 minutes until cooked through.
Toss the contents of the parcel with the hot pasta. Add the spinach and stir until it wilts. Add the lime juice and season with salt, then serve the pasta immediately with lime wedges.