Preheat your oven to 425°F while you prepare ingredients. Pat the chicken breasts dry with paper towels—this ensures they brown nicely rather than steam. Cutting the potatoes into halves or quarters ensures they'll finish cooking in 20 minutes alongside the chicken.
Arrange chicken breast in the center of a large sheet pan, then scatter baby potatoes around it. The potatoes benefit from proximity to the chicken, which protects them from drying out. Leave space between pieces so hot air circulates evenly.
Whisk together olive oil, garlic powder, paprika, Italian seasoning, salt, and black pepper in a small bowl. Add lemon juice and Dijon mustard to the mixture, stirring until smooth. This is your magic seasoning paste for the back to school sheet pan dinner.
Brush half the lemon-Dijon mixture over the chicken and potatoes, making sure everything gets coated. Pour chicken broth around the pan's edges—not over the chicken, which would prevent browning. The broth steams the vegetables while the pan creates golden, caramelized edges.
Roast for 12 minutes, then carefully pull the sheet pan out and add broccoli florets and red bell pepper pieces. Toss the vegetables with the remaining sauce, distributing it evenly. Return to the oven for 8 more minutes until the chicken reaches 165°F internal temperature.
Remove from the oven and sprinkle fresh parsley over everything. Let it rest for 2-3 minutes before plating so the juices redistribute through the sheet pan dinner. Taste and adjust seasoning if needed.