Place a slice of provolone cheese on both top and bottom pieces of ciabatta bread. Layer 3 slices pancetta and 1/4 cup spinach over the 4 bottom slices of bread. Divide tomato, bell peppers, and onion equally among the sandwiches. Drizzle each sandwich with about 1 teaspoon each olive oil and vinegar; season with salt and pepper. Replace tops of ciabatta rolls and brush with the remaining olive oil.
Place sandwiches on the panini grill; close the press tightly on top. Cook until golden brown and cheese is melted, about 5 minutes.