Heat 1 tbsp. oive oil in a large skillet over medium-high heat. Salt and pepper chicken. Cook in skillet, turning once, until juices ru clear about 12 minutes. Slice into thin strips and place in large bowl. In separate bowl whisk together sat and pepper, garlic, and vinegar. Slowly whisk in remainin olive oil. Stir in shallot, tomatoes, and basil. Spoon 1/4 cup of the dressing over chicken and toss to coat. Toss spinach with remianing dressing and divide among 4 serving plates; top evenly with chicken and avocado slices.