Trim and blanch haricots verts; place in an 11- x 7-inch baking dish. Heat canola oil in a large skillet over medium-high. Add sliced fresh mushrooms and chopped onion; cook until browned, 6 to 8 minutes. Add all-purpose flour; cook, stirring constantly, 1 minute. Add unsalted chicken stock, heavy cream, black pepper, and kosher salt; cook, stirring, until thick and smooth, 1 to 3 minutes. Spoon over haricots verts; sprinkle with whole-wheat panko and grated Parmesan cheese. Spray with cooking spray; broil until golden brown, 1 to 2 minutes.