Mix together the cream cheese and scallions until well combined.
Melt the butter in a large skillet over medium heat. Add the bread slices and toast until lightly browned on one side; remove to a cutting board, toasted-side up (the toasted sides will become the insides of your sandwich). Spread all four slices with scallion cream cheese, then sprinkle with Everything Bagel Seasoning. Top two of the slices with mozzarella, sliced tomato and red onion. Press the capers into the cream cheese on the other two slices. Form sandwiches.
Add a bit more butter to the skillet if it seems dry. Place the sandwiches in the skillet, cover with a lid and cook until toasted on the bottom, 3 to 5 minutes. Flip the sandwiches and cook, covered, until toasted on the reverse side and the mozzarella is melted, about another 2 minutes. Sprinkle with a pinch of salt, cut in half and serve.
Everything Bagel Seasoning:
Mix together the sesame seeds, poppy seeds, dried minced garlic, dried minced onion and salt.