Marmitako

Saute one thinly sliced onion in olive oil about 4 minutes.  Add 4 peeled plum tomatoes and 3 garlic cloves.  Simmer 10 minutes.  Add 1/4 cup drained capers.  Peel and slice 4 potatoes 1/2 thick.  Place in bottom of casserole.  Pour the tomato mixture on top.  Add 2 cups stock.  Cook at 400 degrees for 20 minutes.  Add 1 pound tuna and cook 7 more minutes.  Season with salt and pepper.  Thanks, Enrique!