Step 1: Cook the pasta. Cook spaghetti in a large pot of plenty of salted water according to package directions until al dente. Drain well. Rinse immediately under cold water to stop cooking and cool quickly. Let cool completely.
Step 2: Make the dressing. In a small bowl or jar with a lid, whisk together the olive oil, red wine vinegar, Dijon mustard, minced garlic, dried Italian seasoning, salt, and pepper until well combined. If using a jar, shake well.
Step 3: Combine ingredients. In your biggest mixing bowl, add the completely cooled spaghetti, halved cherry tomatoes, diced cucumber, diced red bell pepper, sliced black olives, thinly sliced red onion, mozzarella pearls (or diced), grated Parmesan cheese, and chopped fresh herbs.
Step 4: Add the dressing. Pour the dressing over the ingredients in the bowl.
Step 5: Toss. Gently toss everything together using tongs or two forks, lifting from the bottom to ensure all ingredients are coated in dressing.
Step 6: Chill. Cover the bowl and refrigerate for at least 30 minutes, preferably an hour or two, to allow flavors to meld.
Step 7: Serve. Before serving, give the salad another quick toss. Taste and adjust seasoning with salt or pepper if needed. Garnish with extra fresh herbs or Parmesan if desired. Serve immediately.