In a small bowl, whisk together soy sauce, rice wine vinegar, brown sugar, garlic, ginger, sesame oil and Siraracha (optional). Set aside.
In a large pot, boil some water. Cook udon noodles according to package instructions. Drain well.
In a large skillet, heat olive oil over medium-high heat. Add ground beef and onion. Cook until beef has browned, about 5 minutes.
Stir in bell peppers and broccoli. Cook, stirring frequently until vegetables are tender, about 4-5 minutes. Add udon noodles and soy sauce mixture until well combined, about 2-3 minutes.
Serve immediately. Garnish with green onions.