Set the butter out to soften. Wash the spinach if not pre-washed. Slice the onion into rings.

In a large cast-iron or other heavy skillet, saute spinach in oil until wilted, about 2 minutes. Drain; wipe skillet clean.

Spread 4 bread slices with salad dressing. Layer with a cheese slice, chicken, spinach, onion and another cheese slice. Top with remaining bread. Butter outsides of sandwiches.

Cook in same skillet or panini maker until bread is golden brown and cheese is melted.