Preheat the oven to 375°F.

Combine peanut butter, soy sauce, chicken broth, melted butter, Sriracha, garlic, sesame oil, sesame seeds, and red pepper in a bowl. Whisk until thoroughly combined.

In a separate bowl, whisk together the flour and water to create a slurry.

Over medium-high heat, bring the peanut butter mixture to a slight boil. Slowly add the flour and water slurry, whisking to combine, until the sauce is slightly thickened. Remove from heat.

Place the flank steak on top of a silicone roasting rack on a jelly roll pan. Sprinkle with salt and pepper.

Arrange the carrots and quartered onion around the steak.

Using a flavor injector, fill the injector tube with the sauce and inject the steak.

Bake for 35-45 minutes, until the steak is cooked and the vegetables are roasted.

Drizzle the remaining sauce over the steak and vegetables. Garnish with crushed peanuts and scallions.

Serve over cooked vermicelli, if desired.