Get a large sheet of tin foil (big enough to wrap the fish well). Lay the fennel fronds in the middle of it. Place the fish on top, lying them next to each other.
Put on slice of lemon in the cavity of each fish, and place the other two slices on top of the fish. Scatter the whole garlic cloves around the fish. Sprinkle with the salt. Wrap the tin foil tightly around the fish, then store in the fridge while you prepare the barbecue.
Bring the barbecue up to heat (on a charcoal barbecue, this will be when the coals have turned white). Place the foil parcel on the barbecue and cover with a lid if you have one. Cook for 30 minutes. Serve immediately.
Video
Notes
To cook in the oven instead, preheat your oven to 180 degrees Celsius. Put the foil parcel on a baking tray and place in the oven. Cook for 30 minutes, until the fish is opaque.