Make the dressing: combine 1/2 cup avocado or olive oil, 1/4 cup cider vinegar, 2 Tbsp lemon juice, minced garlic, 1 Tbsp dried oregano, 1/2 tsp ground cumin, 1/2 tsp dried marjoram (or sage or thyme), and 1/2 tsp sea salt in a small blender and blend until smooth and creamy.
Marinate the chicken: place chicken breasts in a sealable bag or container with 2 Tbsp oil, 2 Tbsp lemon juice, 2 tsp dried oregano, 2 tsp dried basil, 2 tsp dried parsley, 1 tsp dried rosemary, and 1/2 tsp sea salt; seal and toss to coat. Marinate at least 15 minutes or up to 8 hours in the refrigerator.
Preheat the oven to 350°F (175°C) and transfer the marinated chicken to a casserole or baking dish, arranging in a single layer.
Bake the chicken on the center rack for 25 to 30 minutes, or until cooked through and an instant-read thermometer registers 165°F (74°C).
Rest and slice: remove the chicken to a cutting board and let rest 10 minutes, then chop into bite-sized cubes (chill first if you prefer cold chicken for the salad).
Assemble the salads: divide the baby spinach, halved cherry tomatoes, sliced red onion, sliced cucumber, sliced avocado, crumbled feta, and toasted pine nuts among four bowls.
Top each salad with the herbed roasted chicken and drizzle with the prepared Greek dressing; serve immediately.