Heat oven to 425°F. Toss potatoes and green beans with 2 tbsp olive oil, ½ tsp garlic powder, ½ tsp salt, and ¼ tsp pepper on a large rimmed baking sheet; spread into a single layer.

Combine Parmesan, panko, oregano, remaining ½ tsp garlic powder, ½ tsp salt, and ¼ tsp pepper in a shallow dish.

Pat chicken dry, brush all over with Dijon mustard, then press the Parmesan mixture firmly onto both sides. Arrange chicken among the vegetables on the sheet pan.

Drizzle the chicken lightly with olive oil and roast 25 to 30 minutes, until the chicken reaches 165°F and the crust is deep golden.

Squeeze lemon juice over everything and serve hot.