Heat the vegetable oil in a large skillet or wok over medium-high heat. Add the ground beef and cook, breaking it up with a spoon, until browned. Drain off any excess grease.
Add the chopped onion to the skillet with the beef. Cook for 3-4 minutes until softened and translucent. Stir in the minced garlic and grated ginger, cooking for another minute until fragrant.
Toss in the sliced red bell pepper and broccoli florets. Stir-fry for 3-5 minutes until the vegetables are tender-crisp. You want them to still have a slight bite.
While the vegetables are cooking, prepare the peanut sauce. In a small bowl, whisk together the peanut butter, soy sauce, rice vinegar, honey (or maple syrup), sriracha, and water (or beef broth) until smooth. If using cornstarch, whisk it in now.
Pour the peanut sauce over the ground beef and vegetable mixture in the skillet. Stir well to coat everything evenly. Bring the sauce to a simmer and cook for 2-3 minutes, stirring constantly, until the sauce thickens slightly.
Season with salt and black pepper to taste. Be mindful of the salt from the soy sauce.
Serve the spicy peanut butter ground beef mixture immediately over hot cooked rice or noodles. Garnish with chopped peanuts, fresh cilantro, and sesame seeds, if desired.