Cook the rice noodles according to package instructions and set aside.
In a large skillet or wok, cook the ground beef over medium-high heat until browned, breaking it up into small pieces as it cooks.
Add the garlic and cook for an additional minute.
Add the red onion, red bell pepper, and bean sprouts to the skillet and cook for 3-4 minutes or until the vegetables are tender.
Add the baby spinach and cherry tomatoes to the skillet and cook for an additional minute or until the spinach has wilted.
In a small bowl, whisk together the fish sauce, brown sugar, lime juice, and chili flakes (if using).
Pour the sauce over the beef and vegetables in the skillet and toss to combine.
Add the cooked rice noodles to the skillet and toss to combine.
Garnish with cilantro, mint, and peanuts (if using) before serving.