Preheat oven to 350°F (180°C). Melt 2 tbsp margarine in large saucepan over medium heat. Whisk in flour blend until smooth; cook, whisking constantly, until thick paste forms, about 2 minutes.
Gradually whisk in 1 cup soy beverage until smooth. Whisk in remaining 1-½ cups soy beverage, mustard, vinegar and salt until smooth. Cook, stirring constantly with wooden spoon, until mixture is consistency of pudding, 4 to 5 minutes. Remove from heat. Set aside
Bring 4 L (16 cups) water to a boil in separate large saucepan. Add frozen squash; cook 3 to 5 minutes. Transfer squash with slotted spoon to food processor or blender; add soy beverage mixture and purée until smooth. Set aside.
Return same saucepan of water to a boil. Add pasta; stir until water returns to a boil. Cook 1 minute. Add broccoli; return to a boil. Cook, stirring occasionally, until pasta is tender but firm and broccoli is tender-crisp, about 2 minutes. Drain and rinse with cold water until chilled. Return to saucepan.
Meanwhile, melt remaining 2 tbsp margarine. Stir together melted margarine and bread crumbs in small bowl. Set aside.
Add squash mixture, cheese and nutritional yeast to saucepan with pasta mixture; stir until well combined. Transfer to 13 x 9-inch (3 L) glass baking dish. Sprinkle with bread crumb mixture. Cover with foil.
Bake until bubbling, 30 to 35 minutes. Remove foil; bake 10 minutes. Let stand 15 minutes before serving.
Chef's Tip: Scrape the corners of the saucepan with your whisk or spoon while the sauce is cooking – this prevents any flour from sticking and burning on the bottom of the pan. And substitute frozen peas for the broccoli, if you prefer.
Nutrients per serving: 40 calories, 13 g fat (saturated fat 6 g), 570 mg sodium, 43 g carbohydrate, 2 g fibre, 4 g sugars, 13 g protein.