Place chicken a large Ziploc baggie with 1/3 cup balsamic vinegar, lemon zest and juice, oregano and ½ tsp salt and ¼ tsp black pepper. Seal bag and gently toss (over the sink – yes, I’ve learned the hard way!) to combine. Marinade for 30 minutes, up to overnight. Pull out of the fridge 30 minutes prior to cooking.
Preheat the oven to 425 degrees F and line two small sheet pans with foil.
While oven is heating up, on one sheet pan, place the tomatoes and on the other sheet pan, lay the sourdough out, in an even layer. Drizzle the tomatoes and sourdough with ½ tbsp olive oil each and sprinkle both with salt and black pepper. Set the croutons aside and place the tomatoes into the oven to roast for 7-10 minutes until slightly browned and starting to burst open.
Using tongs, pull the chicken out of the bag and place each piece in an even layer, without touching, onto a large nonstick skillet over medium heat; discard the remaining marinade. Let cook for 2-3 minutes per side, until each piece is cooked through.
Meanwhile, in a large salad/serving bowl place spinach, basil, goat cheese and avocado.
When you’re ready to pull the tomatoes out of the oven, place sourdough on the top rack of the oven to toast for 2-3 minutes and pour the tomatoes, while hot, in with the spinach and then add the chicken.
Drizzle remaining olive oil and balsamic over the top and toss gently to combine and coat with the dressing. Sprinkle with salt, black pepper and red pepper flakes (if using) and place croutons on top. Toss again to combine and serve!