Preheat the oven: Set your oven to 425°F (220°C). Line a large rimmed sheet pan with parchment for easy cleanup, or lightly oil it.
Prep the veggies: Halve the baby potatoes and slice the carrots.
Cut broccoli into small florets and the red onion into wedges. Aim for bite-sized, even pieces so everything cooks at the same rate.
Cut the chicken: Slice chicken into 1.5-inch chunks. Pat dry with paper towels to help browning.
Season everything: In a large bowl, toss potatoes, carrots, broccoli, onion, and any optional veggies with 2 tablespoons olive oil, 1 to 1.5 tablespoons ranch seasoning, a pinch of salt, and pepper.
In a separate bowl, toss chicken with the remaining 1 tablespoon olive oil, 1 to 1.5 tablespoons ranch seasoning, minced garlic, and a light pinch of salt and pepper.
Arrange on the pan: Spread the potatoes and carrots on the pan first since they take longer. Scatter the onion and broccoli around them, leaving space for the chicken. Add the chicken pieces in a single layer, not touching if possible.
Roast: Bake for 15 minutes.
Remove the pan, gently toss or flip the chicken and veggies, then return to the oven for another 10 to 15 minutes, until the chicken is cooked through and the vegetables are tender with caramelized edges.
Check doneness: Chicken should reach an internal temperature of 165°F (74°C). Potatoes should pierce easily with a fork.
Finish and serve: Squeeze fresh lemon over everything and sprinkle with chopped parsley or chives. Taste and adjust seasoning with extra ranch, salt, or pepper if needed.
Serve hot.