Lay 4 slices of the bread on a board and spread each slice with the mustard. Top the slices with half of the Gruyère or Cheddar, the turkey and tomato slices, then scatter with the spring onions. Season with pepper and scatter over the remaining Gruyère or Cheddar. Spread the cream cheese, if using, over the remaining slices of bread and place, cheese side down, on top of the sandwiches.
Heat a large, nonstick frying pan over a medium heat until hot, then carefully add the sandwiches and cook for 4–5 minutes or until golden and crispy. Turn the sandwiches over and cook for a further 4–5 minutes. Alternatively, toast in a flat-surfaced panini machine according to the manufacturer's instructions.
Meanwhile, bring a large saucepan of water to a gentle simmer, add the vinegar and stir with a large spoon to create a swirl. Carefully break 2 eggs into the water and cook for 3 minutes. Remove with a slotted spoon and keep warm. Repeat with the remaining eggs.
Transfer each sandwich to a serving plate, scatter over a few spinach leaves and top with a poached egg. Garnish with chives and serve immediately.