In a medium jar, combine olive oil, balsamic vinegar, Dijon mustard, lemon juice, granulated sugar, kosher salt, and black pepper. Secure the lid tightly and shake vigorously until emulsified.

Preheat your oven to 425°F (220°C). Halve the cherry tomatoes and place them on a baking sheet lined with parchment paper. Drizzle with olive oil and sprinkle with salt. Roast for about 15 minutes until tender.

Bring a large pot of salted water to a rolling boil. Add short pasta and cook according to package instructions until al dente, usually about 8-10 minutes. Drain and rinse with cold water.

In a large mixing bowl, toss the warm pasta with the prepared dressing. Gently fold in the roasted tomatoes, arugula, drained tuna, dill, and chives. Mix gently to combine.

Cover the bowl with plastic wrap or an airtight container and refrigerate for at least 2 hours to allow flavors to meld.

Before serving, toss the salad gently to redistribute flavors. Add fresh arugula on top and sprinkle with flaky sea salt if desired.