Heat a large skillet or wok over medium-high heat, adding the sesame oil.
Once the oil is hot, add the sliced onion and minced garlic, and sauté for about 2 minutes until fragrant and the onion starts to soften.
Add the ground beef to the skillet, breaking it up with a spatula. Cook for about 5-7 minutes until browned and cooked through.
Stir in the mixed vegetables and cook for another 3-5 minutes until the vegetables are tender but still vibrant.
Add the soy sauce, oyster sauce (if using), minced ginger, salt, and pepper. Stir everything to combine and cook for an additional 2 minutes to allow the flavors to meld.
Remove from heat and serve over cooked rice or noodles.
Feel free to customize the vegetable mix to your preference or use whatever you have on hand.
For added spice, consider adding a splash of sriracha or red pepper flakes.
This dish can be made ahead of time and reheated for a quick dinner.