For Onion Relish: toss onion wedges and julienne peppers with Italian dressing, basil and pepper. Marinate in refrigerator overnight.
For Focaccia: roll out dough to fit bottom of 15 x 10-inch pan. Place dough in oiled pan, oil top and set in warm place until it rises about 1-inch in pan. Oil top again and sprinkle with minced onions and thyme. Make impressions in dough about 1-inch apart. Bake on high rack at 400 degrees 16 to 20 minutes or until golden.
Cut 8 squares and split. Fill squares with spinach, tomatoes and roast beef. Top with onion relish. Makes 8 servings.