Preheat oven to 230°C/450°F, combination steam setting. If your oven has variable steam settings, select 60-80% (if not, don’t worry about this step, your oven will take care of this for you).
Spread the sweet potato chips over one or two baking trays in a single layer (I use my favourite solid stainless steel ones). If your steam oven is a full-size model you may be able to get away with only one tray, just make sure the chips are not really touching one another so they cook properly. Better to use two if you aren’t sure. Drizzle the oil over the chips and sprinkle the semolina (if usinand salt over the top. Toss to combine, then tuck the garlic and thyme sprigs amongst the sweet potato. Place in the preheated oven for 15 minutes.
While the chips start cooking, place the fish fillets into another baking tray. Mix the rest of the ingredients together except the whole lemon and mayonnaise, and heap the crumb mix onto the top of each fillet, gently pressing it down.
When the first 15 minutes is up for the chips, add the fish and set your timer for a further seven to eight minutes, depending on the thickness of your fish. If you’ve used two trays for the chips, swap them over in the oven as the fish goes in, so they cook evenly.
When the fish is cooked, remove everything from the oven, discard herbs and garlic (or save that soft roasted garlic for another disand serve immediately on warmed plates with lemon mayonnaise on the side.