Preheat the oven: Set your oven to 425°F (220°C). Line a large sheet pan with parchment paper for easy cleanup.
Prep the vegetables: Slice zucchini into half-moons about 1/2-inch thick.
Slice bell peppers and red onion into strips. Add them to the sheet pan.
Season the vegetables: Drizzle with 1 tablespoon olive oil. Add 1/2 teaspoon salt and 1/2 teaspoon black pepper.
Toss to coat and spread into an even layer, leaving space for the chicken.
Prepare the chicken: Pat chicken dry with paper towels and cut into 1–1.5-inch pieces or keep as whole breasts if you prefer slicing after cooking. Place in a bowl.
Make the seasoning mix: In a small bowl, combine minced garlic, lemon zest, half the lemon juice, 1 tablespoon olive oil, 1 teaspoon salt, 1/2 teaspoon black pepper, smoked paprika, oregano, cumin, and red pepper flakes if using. Stir into a paste.
Coat the chicken: Pour the seasoning mixture over the chicken and toss until evenly coated.
Add the chicken to the open space on the sheet pan. If the pan feels crowded, use a second pan.
Roast: Bake for 16–20 minutes, flipping the veggies halfway. Chicken pieces should reach an internal temperature of 165°F (74°C).
If using whole breasts, you may need 20–24 minutes depending on thickness.
Finish with lemon: Squeeze the remaining lemon juice over everything right after baking. Sprinkle with chopped parsley or cilantro.
Portion: Divide into 4–5 meal prep containers. Add a small serving of cooked rice, quinoa, or greens if you want extra volume or carbs.