Summery Green Bean Casserole

Preheat oven to 425°F. Mist a 2-qt baking dish with cooking spray.Bring a large saucepan of water to a boil. Add beans and cover until water returns to a boil. Cook, uncovered, until tender-crisp, 5 to 6 minutes. Meanwhile, place a colander in the sink and fill a large bowl with ice water. Drain beans in colander and immediately add to ice water. Let sit for 10 minutes, then drain. (NOTE: Beans may be cooked up to 1 day ahead; cover and refrigerate.)In a large skillet, heat 2 tsp oil on medium-low. Add onion, season with scant 1/16 tsp salt and pepper, and cook for 15 to 20 minutes, stirring occasionally, until soft and golden brown. Transfer onion to a bowl. Coat skillet with cooking spray and bring to medium heat. Add mushrooms, lemon-pepper seasoning and scant 1/16 tsp salt. Cook, stirring often, until golden brown, about 8 minutes; set aside.Add remaining 2 tbsp oil to saucepan used to cook beans and heat on medium-low. Add garlic and cook for 1 minute. Add flour and cook for 1 minute, stirring constantly. Increase heat to high and slowly whisk in milk. Bring to a simmer, whisking frequently. Reduce heat to medium-low and simmer until slightly thick, 4 to 6 minutes. Reduce heat to low and stir in 1/8 tsp salt, coriander, rosemary, thyme, tomatoes, beans and mushrooms. Mix well, adjusting seasoning as desired. Transfer mixture to prepared baking dish and cover evenly with a layer of onion, followed by a layer of panko.Bake until liquid is bubbling, 15 to 20 minutes. Let rest for 10 minutes and serve.