Prepare spinach-rice stuffing.
With kitchen scissors, cut hens in half lengthwise.
Rinse and pat dry.
Press 3/4 cup of the stuffing onto cut side of each hen half.
Holding stuffing in place with your hand, arrange hens, skin side up, in a greased shallow baking pan.
Melt butter in a small pan over low heat; stir in onion, parsley, mustard, and fines herbes.
Brush over hens.
Bake, uncovered, in a 375° oven for 45 to 60 minutes or until leg joint moves easily; during last half-hour, baste occasionally with pan drippings