Rub the fish on both sides with butter

From 1912:

The rules are the same for large and for small fish. The latter, however, are seldom cooked in this way.
Clean and wash the fish and split down the back as for broiling. Have at hand a clean board of oak or hemlock (never of pine) that will fit easily into your oven. Set it in the oven to heat, turning it several times and watching to see that it does not burn.
Rub the fish on both sides with butter – plenty of it – and salt and pepper to taste. Lay the fish with the back downward and the open side uppermost upon the board and put a tin tack into each of the four corners. Lay the plank upon the upper grating of the oven, shut the door, and bake.
Rub it every five minutes with butter until it is done. Test with a fork, and if there is no sign of rawness in the thickest part withdraw the tin tacks carefully and slip the fish upon a hot platter. Send around melted butter with it; garnish with sliced
lemon and parsley sprigs.