In a large bowl, combine olive oil and ranch seasoning mix. Stir well to create a marinade.
Add the chicken breasts to the bowl and coat them thoroughly in the ranch marinade. Set aside for 10-15 minutes to enhance flavor.
On a large baking sheet, arrange the halved baby potatoes, bell peppers, zucchini, and red onion in a single layer. Drizzle with a little olive oil and season with salt and pepper. Toss to coat.
Nestle the marinated chicken breasts among the vegetables on the baking sheet.
Place the baking sheet in the preheated oven and bake for 25-30 minutes until the chicken is cooked through (internal temperature should reach 165°F or 75°C) and the veggies are tender.
Remove from the oven and let rest for 5 minutes before serving.
Garnish with freshly chopped parsley to add a pop of color and flavor before serving.