Marinate seitan in Italian dressing overnight then chop into chunks. Melt margarine and mix together with bread stuffing mix, seitan, onions, celery and veggie broth. Press into the bottom of a sprayed lasagna pan.
Spread leftover mashed potatoes over the bread mixture. Sprinkle with paprika and bake at 350 for 30min.
Allow pan to cool for 15min then cut large circles out of the casserole. Remove pieces around the circles, then carefully remove the circles themselves. Cover with gravy or white sauce and serve warm.