Cut bread in half horizontally. Brush the outside of loaf with oil. Place bottom half of loaf on a piece of parchment or waxed paper. Set top aside. In a small bowl, mix mayonnaise and pesto; set aside.
Preheat an electric sandwich press, covered indoor grill, grill pan, or 12-inch skillet. Pace half of the cheese on bottom half of bread. Spread mayonnaise mixture over cheese. Layer with meats, Red Onion Relish, spinach, and remaining cheese. Replace top. Cut into 4 sandwiches. Place sandwiches (half at a time, if necessary) in the sandwich press or indoor grill; cover and and cook about 6 minutes or until bread is toasted. (If using a grill pan or skillet, place sandwiches on grill pan. Weight sandwiches down and grill about 2 minutes or until bread is toasted. Turn sandwiches, weight down, and grill remaining sides.) Makes 4 sandwiches.
Red Onion Relish
Combine red onion, halved and thinly sliced; olive oil; red wine vinegar; and snipped fresh oregano. Season with salt and ground black pepper. Cover, let stand at room temperature up to 2 hours. Drain.