In a small bowl, whisk the balsamic vinegar with olive oil, honey, smoked paprika, fennel seeds, salt, and pepper, until blended.
Place the chicken breasts in a shallow bowl and pour half of the dressing. Cover and refrigerate for 10 minutes to 2 hours.
Spray a grill pan or non-stick pan with cooking spray and heat to medium-high. Place the chicken breasts on the hot pan. Cook for 3 minutes then flip them. Cook for another 3 minutes, and turn. Reduce the cooking temperature to medium low and cook the chicken for 15 minutes more, turning every 5 minutes or so. Cooking time depends on the chicken's thickness, but it will be fully cooked when it hits 75C/165F degrees internal temperature. Let it rest for 5 minutes then slice it.
Place the spinach, strawberries and red onion in a bowl. Add the avocado, sliced chicken, and top with feta and hazelnuts.
Pour the remaining dressing, mix and serve immediately.