Roll each bread slice flat with a rolling pin and trim it to form a 2 1/2" square. Brush both sides of the squares lightly with the butter and fit th squares into gem muffin tins, pressing the bread against the sides of the tins.
Bake the croustades in the middle of a preheated 350 deg. oven 12-14 minutes, oruntil the edges are pale golden. These may be made 2 days in advance & kept in an airtight container. In a bowl stir together the artich hearts, mayonnaise, 1 tb of the scallion, 1/4 c of the Parmesan, and salt & pepper to taste.
Divide the mixture by heaping teaspoons among the croustad and sprinkle the remaining Parmesan on top. Broil the croustades under a preheated broiler about 6" from the heat, being careful not to let the croustade edges burn, for 1 minute, or until the filling is bubbling.
Garni the croustades with the scallion green and arrange 3 of them on each of 6 small plates.