Preheat the oven to 425°F (220°C). Wash the baby potatoes and halve any larger ones so all pieces are roughly the same size. Peel or scrub the carrots and trim ends. Cut the red onion into 1-inch wedges. Pat the chicken thighs dry with paper towels and season both sides with 1 tsp kosher salt and a pinch of black pepper.

In a large mixing bowl whisk together 3 tbsp olive oil, 2 tbsp fresh lemon juice, minced garlic, 1 tsp Dijon mustard, 1 tsp dried oregano, the remaining 0.5 tsp kosher salt, and 0.25 tsp freshly ground black pepper until emulsified.

Add the chicken, halved potatoes, carrots, and red onion to the bowl and toss thoroughly so everything is evenly coated. Let sit at room temperature for 10 to 15 minutes while the oven finishes heating so flavors meld and the chicken comes slightly closer to room temp.

Spread the chicken and vegetables in a single layer on a rimmed sheet pan (use parchment if you like). Make sure the chicken pieces are skin-side up (if they had skin) or spaced so hot air circulates—crowding prevents browning.

Roast in the preheated 425°F oven for 25 to 30 minutes, stirring the potatoes and carrots once at the 15-minute mark so they brown evenly. Start checking the chicken at 20 minutes; boneless thighs are done when an instant-read thermometer inserted into the thickest part reads 165°F and the potatoes are tender when pierced with a fork.

If you want extra browning, switch the oven to broil for 2 to 3 minutes, watching closely so the garlic and veggies don't burn. Remove from oven when chicken reaches 165°F and edges of potatoes are golden.

Let the pan rest 5 minutes so juices settle. Sprinkle chopped fresh parsley and a squeeze of lemon over everything. Serve directly from the pan or transfer to a platter with lemon wedges alongside.