Heat the olive oil in a large skillet or wok over medium-high heat until it shimmers, about 1 minute.

Add the ground beef and cook for 5-6 minutes. Break it up with a wooden spoon until browned and no pink remains; drain excess fat if needed.

Add the sliced red onion, bell peppers, and zucchini. Cook for 5-7 minutes, stirring occasionally, until the vegetables are tender-crisp and have a slight char.

Stir in the minced garlic, dried oregano, dried thyme, and red pepper flakes. Cook for 1 minute, stirring constantly until fragrant but not browned.

In a small bowl, whisk together the chicken broth, lemon juice, fresh parsley, and Dijon mustard. Pour this sauce over the beef and vegetables, stirring to combine.

Cook for 2-3 minutes until the sauce thickens slightly and coats everything in a glossy finish. Season with salt and black pepper to taste.

Serve immediately, garnished with additional fresh parsley if desired.