1. Whisk the extra virgin olive oil with the balsamic vinegar, sugar, tarragon, kosher salt and freshly ground black pepper in a small bowl until blended.
2. Place the chicken breasts in a shallow bowl and cover with half of the dressing, cover and refrigerate for 30 minutes to 2 hours.
3. Spray a grill pan or 12-inch non-stick pan with cooking spray and heat to medium high. Place the chicken breasts on the hot grill pan. Cook for 3 minutes then flip the chicken breasts. Cook for another 3 minutes, and turn. Reduce the cooking temperature to medium low and cook the chicken for 20-25 minutes more, turning every 5 minutes or so. Cooking time will depend on the thickness of the chicken, but it will be done when it hits 165 degrees internal temperature. Let the chicken rest for 5 minutes then slice into ¼ inch slices.
4. Arrange the spinach, strawberries and red onion in a bowl. Lightly toss with the remaining dressing. Add the avocado, sliced chicken and top with feta and almond slices. Serve immediately.