Vegan Green Bean Casserole

Name: Vegan Green Bean Casserole
Ingredients: 2 sweet onions , halved and thinly sliced, divided; Canola oil , for frying; 1/4 cup cornstarch; Kosher salt , plus more for seasoning; 1 pound green beans , trimmed and cut into 2-inch pieces; 2 tablespoons extra virgin olive oil; 1 clove garlic , minced; 1 cup thinly sliced shiitake mushrooms; Freshly ground black pepper; 3 tablespoons all-purpose flour; 1 cup vegetable broth; 1 cup full-fat coconut milk; 1 teaspoon soy sauce , plus more to taste