Position rack in center of oven and preheat the oven to 450°F. Brush the peppers with 1 tablespoon of the olive oil. Place them on an ungreased baking sheet and roast for 1 hour.
Using oven mitts or pot holders, remove the peppers from the oven and place them in a large mixing bowl. Cover with plastic wrap, and let them steam until cooled, about 30 to 40 minutes. Do not handle the hot peppers with your bare hands!
When cool, peel the skin from the peppers. Cut or tear open the peppers so that they lay flat, and remove the seeds, but try to keep each pepper in one piece. Set aside.
To make the sandwiches, slice each 4 by 5-inch piece of focaccia in half horizontally. Be careful here!
Separate the 16 pieces into 8 "top" pieces and 8 "bottom" pieces.
In a small mixing bowl, combine the remaining 3/4 cup of olive oil, balsamic vinegar, salt, and pepper. Whisk to blend. Spread one tablespoon of this mixture on the cut side of each piece of bread.
Divide the mozzarella slices evenly among the 8 bottom halves, then top each half with 1 bell pepper and 1/2 cup of the spinach. Drizzle the remaining olive oil mixture over the spinach, and then place the top half of the bread over the spinach. Slice the sandwiches in half diagonally to serve.
YIELD: 8 sandwiches