Preheat oven to 400 degrees F. Place both halves of the baguette on a large baking sheet, cut sides up.
In a medium bowl, stir together softened butter and garlic. Spread evenly on bread halves. Completely cover the top of the bread with slices of mozzarella cheese. Bake for 12-15 minutes or until the cheese is melted, turning the broiler on high for final 2 minutes to brown the cheese slightly.
While the bread is in the oven, make the pesto. In a food processor, combine the pecans, garlic, arugula, cheese, and 1 teaspoon of salt; pulse to blend.
With the machine running, drizzle in the olive oil in a slow, steady stream and blend until smooth; scrape down the sides of the bowl as needed. Taste and adjust salt as desired.
After removing the bread from the oven, top with desired amount of pesto, then layer on tomato slices. Season with salt and pepper to taste and add the fresh basil. Drizzle with balsamic vinegar, slice, and serve.