Preheat the oven to 200°C/ gas 6, and preheat the grill.
In the preheated oven, roast the sweet potatoes in a little olive oil until golden, about 20 minutes. Heat all the ingredients for the glaze in a small pan and let them bubble to thicken slightly, stirring constantly.
Line a grill pan with foil, then season the mackerel all over and place skin side down.
Brush with the glaze and grill for 2 minutes.
Turn the fillets skin side up, brush with the glaze and grill until the skin just begins to blister and the fish is opaque and cooked through.
Serve with the sweet potatoes, some green beans and sugar snap peas, and the juices from the grill pan spooned over the top.