MIX dressing and seasoning; spread evenly onto bread slices.
FOR each serving: Top 1 of the bread slices with 2 oz. ham, 2 tomato slices, 1 onion slice and 2 process cheese slices. Cover with second bread slice, dressing side down. Brush outside of sandwich with butter.
GRILL on heated panini grill 4 to 5 minutes or until golden brown. Serve warm.