For the Tomatoes:

1. Preheat oven to 450°F.
2. Line a baking tray with parchment paper. Arrange the tomatoes on the tray.
3. Drizzle with olive oil. Sprinkle salt and garlic powder on the tomatoes.
4. Place in the oven and bake until the tomatoes begin to shrivel and have black edges, about 25 minutes.

For the Sandwich:

1. Preheat oven to 350°F.
2. Cut two pieces of focaccia, 6-inches by 3 inches for two sandwiches. Slice horizontally.
3. Spread a tablespoon of pesto on each piece of bread.
4. Spread a few spinach leaves (5 to 10) on two of the bread halves.
5. Place slices of mozzarella on the other two bread halves and top with tomatoes.
6. Assemble each sandwich and wrap each in a piece of parchment paper.
7. Place in the oven and heat until the cheese is melted, 5 to 10 minutes.