Boil the pasta in salted water for 8 to 10 minutes, or until al dente. Meanwhile, whisk together the balsamic vinegar, oil, mustard, salt and pepper flakes in the bottom of a large bowl. Add the tomatoes, mozzarella, salami, onion and basil on top of the dressing.
When the pasta is cooked, drain it and add it to the large bowl. Toss to combine. Serve warm, at room temperature or cool.