Bring a large pot of salted water to a boil. Cook orzo according to package directions until al dente. Drain and rinse with cold water. In a small bowl, whisk together all vinaigrette ingredients: extra virgin olive oil, lemon juice, Dijon mustard, minced garlic, salt, and pepper.

Pat chicken breasts dry and rub with 1 tbsp olive oil, salt, and pepper. Preheat a grill or grill pan to medium-high heat. Grill chicken for 6-8 minutes per side, until cooked through with dark char marks. Let it rest for 10 minutes before slicing.

While chicken rests, chop the cucumber and dice the avocados. Season avocado with a pinch of black pepper. In a large bowl, combine the cooled orzo and chopped cucumber. Pour about three-quarters of the vinaigrette over and toss to coat.

Arrange a bed of arugula in shallow bowls. Top with the orzo-cucumber mixture. Neatly arrange the sliced grilled chicken and diced avocado in distinct sections. Drizzle with the remaining dressing and serve immediately.