Heat the oven: Preheat to 425°F (220°C). Line a sheet pan with parchment for easy cleanup.
Make the marinade: In a small bowl, whisk 1.5 tablespoons olive oil, the zest and juice of half the lemon, garlic, Dijon, oregano, red pepper flakes, 1/2 teaspoon salt, and several grinds of pepper.
Marinate the chicken: Pat the chicken dry.
Add half the marinade to the chicken and coat well. Let sit while you prep the veg (about 10 minutes). Reserve the remaining marinade for later.
Prep the veg: Toss zucchini, squash, bell pepper, red onion, and tomatoes with the remaining 1/2 tablespoon olive oil and a pinch of salt and pepper.
Arrange on the pan: Spread the vegetables in an even layer with a little space between pieces.
Place the chicken on top or nestle it in the center.
Roast: Bake for 16–20 minutes, until the chicken reaches 165°F (74°C) in the thickest part and the veg is tender with caramelized edges. If your chicken is thick, it may need a few extra minutes.
Finish with brightness: Rest the chicken on the pan for 3–5 minutes. Drizzle the reserved marinade over the chicken and vegetables.
Squeeze the remaining lemon half over the top if you like extra zing.
Serve: Slice the chicken and scatter fresh parsley or basil. Taste and adjust salt, pepper, or lemon. Plate as is, or spoon over rice, couscous, or alongside crusty bread to soak up the juices.