Tex-Mex Summer Squash Casserole

2 1/4 pounds summer squash, quartered lengthwise and thinly sliced crosswise (about 10 cups)
2/3 cup finely chopped yellow onion
1 4-ounce can chopped green chiles
1 4-1/2-ounce can chopped jalapenos (about 1/2 cup) drained
1/2 tsp salt, or to taste
2 1/4 cups grated extra-sharp cheddar cheese, (about 7 ounces) divided
1/4 cup all-purpose flour
3/4 cup mild salsa
4 scallions, thinly sliced, for garnish
1/4 cup finely chopped red onion, for garnish

1. Preheat oven to 400 degrees. Coat a 9X13 inch baking dish with cooking spray.
2. Combine squash, onion, chiles, jalapenos, salt and 3/4 cup cheese in a large bowl. Sprinkle with flour; toss to coat. Spread the mixture in the prepared baking dish and cover with foil.
3. Bake the casserole until it is bubbling and the squash is tender, 35-45 minutes. Spoon salsa over the casserole and sprinkle with the remaining 1 1/2 cups cheese. Bake, uncovered, until golden and heated through, 20-30 minutes. Sprinkle with scallions and red onion.