Mix marinade ingredients together. Rub on flank steak, and let the flank rest at room temperature for 1 hour.
Preheat a grill pan over medium heat. Place flank steak on hot pan. Grill for 10 minutes per side, or until a meat thermometer inserted into the thickest part of the flank reads 125°F. Let flank steak rest for 10 minutes before slicing thinly against the grain.
Preheat oven to 325°F
Mix all ingredients for the tomatoes together. Put tomatoes flesh-side down on wire rack laid on top of baking sheet. Slow roast for 1 hour. Cool to room temperature.
Combine vinegar, chives, salt, pepper and mustard in a food processor. Pulse until mixed.
On low speed, add avocado oil in a slow, steady drizzle.
Toss all salad ingredients including oven roasted tomatoes in salad bowl with vinaigrette. Serve with flank steak and enjoy!