Another Portuguese here. As a preference I'd also sprinkle with rock salt and grill over charcoal along with some peppers (green ones traditionally, although it's an acquired taste to British palates) and serve with a green salad and either sourdough bread or boiled potatoes. Throw on some chopped garlic and parsley right at the end of cooking if you want.
British barbecue recipe books suggest gutting them before cooking, but no self-respecting Portuguese would bother doing that. Scale them though, if this hasn't been done for you, and eat the skin.
If there's no access to a bbq, then I prefer use a ridged grill pan on the hob, because the extractor tends to deal with the aroma better than if I grilled using the thing in the oven. They can also be panfried - my mother used to dredge them through seasoned flour if doing this.
If the oven is going on anyway then that is also an option. This works well with a marinade of olive oil, chopped garlic, and lemon.
Another serving suggestion which most of my regular sardine guests like is a salsa of chopped sweet pepper and avocado with parsley.
I believe the traditional Cornish way of cooking and serving them is the Stargazy Pie. Personally I couldn't think of a worse way to waste sardines, but each to their own.