1. Preheat oven to 400 degrees Fahrenheit (200 degrees Celsius).
2. Toss tomato and onion slices with one tablespoon teriyaki sauce. Set aside.
3. Rinse salmon and pat dry. Place in a pan and season with salt, pepper, and garlic powder.
4. In a bowl, mix together mayonnaise, remaining two tablespoons teriyaki sauce, lemon juice, and garlic cubes. Season with salt and pepper, to taste. Smear the mixture all over the salmon, getting the sides as well.
5. Arrange a single layer of tomatoes and onions on the salmon, alternating between vegetables. Once the salmon is covered, cover the vegetables with a layer of mozzarella cheese. Sprinkle oregano over the top.
6. Place in the oven for 22–25 minutes, depending on how well done you like your salmon.