Adjust the oven rack to the middle position.
Preheat the oven to 425 degrees F.
Bring a medium pot of salted water to a boil.
In a small bowl, combine the panko, the parmesan, the paprika, 2 teaspoons of the olive oil, the salt, and the pepper.
Pat the chicken dry with paper towels and season them with the salt and the pepper.
Evenly brush the tops of the chicken with the sour cream.
Mound the panko mixture on top of the chicken breasts, pressing it firmly to adhere. You do not need to coat the undersides of the chicken.
On one side of a rimmed baking sheet, toss the green beans with the remaining olive oil, the salt, and the pepper.
Place the chicken, crust-side up, on the empty side of the same sheet.
Roast until the green beans are tender and the chicken is cooked to 165 degrees F internally, about 15-17 minutes. If the chicken needs more time, transfer the green beans to a plate and let the chicken continue roasting.
Once the salted water is boiling, add the couscous to the pot.
Cook the couscous until tender, about 6-8 minutes.
Drain the couscous thoroughly.
In an empty pot over medium heat, melt the garlic herb butter.
Add the scallion whites to the butter and cook until softened, about 1 minute.
Add the couscous to the butter and scallion mixture, stirring to coat.
Taste the couscous for seasoning.
Toss the green beans with the lemon zest and lemon juice.
Divide the chicken, the couscous, and the green beans between two serving plates.
Serve garnished with the green parts of the scallions.