Shredded Chicken-Enchilada Casserole

2 spray(s) cooking spray

1 ¹/₂ Tbsp olive oil

3 cup(s), chopped uncooked onion(s), divided

1 pound(s) cremini mushroom(s), sliced 1/4-in thick

2 medium uncooked zucchini, halved lengthwise, sliced 1/4-in thick

1 ¹/₂ tsp table salt, divided

28 oz canned diced tomatoes, fire-roasted (undrained)

1 item(s) canned chipotle peppers in adobo sauce, chopped (optional)

4 clove(s), medium garlic clove(s), smashed

2 tsp chili powder

2 tsp ground cumin

1 tsp dried oregano

12 oz cooked boneless skinless chicken breast(s), shredded with two forks

18 small corn tortilla(s), 4-in each

6 oz low fat shredded cheddar cheese, or jack cheese

³/₄ cup(s) reduced-fat sour cream