Make the salad dressing. Combine ⅓ cup extra virgin olive oil, 2 Tbsp balsamic vinegar, 2 Tbsp Dijon mustard, ½ a lemon juiced, 1 Tbsp sugar, ½ tsp Kosher salt, and ¼ tsp black pepper in a measuring cup, then whisk to combine. (You can also place everything in a mason jar, seal tightly, then shake.) Set aside.
Roast the tomatoes. Preheat an oven to 425F. Place the tomatoes on a rimmed baking sheet, then drizzle with 2 Tbsp olive oil and ½ tsp Kosher salt, then roast for 15 minutes.
Cook the pasta. Bring a large pot of salted water to a rapid boil, then add ½ lb pasta. Cook according to package directions until just past al dente, or until pasta is tender, but not mushy. Drain the pasta.
Toss it all together. Place pasta in a large mixing bowl, then pour balsamic vinaigrette on top. Add roasted cherry tomatoes, 2 heaping cups of arugula, the drained tuna (add it in large chunks, if possible!), 2 Tbsp chopped dill, and 2 Tbsp chopped chives. Carefully toss to mix well.
Chill, then serve. You can certainly enjoy this right away, but it's even better chilled for at least 2 hours. Can be served cold or at room temperature. Just before serving, add in another handful of baby arugula (if you want some that hasn't wilted) and sprinkle with a tiny bit of flaky sea salt.