Preheat oven to 350 degrees and move a rack to the top. Boil water for pasta.
Simmer corn, milk, and onion powder. In small bowl, add cornstarch to 2 Tbsp water to dissolve. Add cornstarch mixture to corn and whisk over medium heat to thicken, around 3-5 minutes. Season with salt and pepper. Meanwhile, cook pasta and drain thoroughly.
Combine corn mixture, pasta, cheddar cheese, egg, and half the mozzarella. Pour into 4 cup casserole dish. Top with remaining mozzarella.
Toss croutons with olive oil and spread on the casserole. Cook on the top rack until golden and bubbly, 15-20 minutes. Drizzle with Sriracha or hot sauce to serve (or serve on the side)