Preheat oven to 425. Bring a medium pot of salted water to a boil. Cut peeled carrots into quarters ~ 2" long. Thinly slice scallions - separating greens and whites. Zest and quarter lemon.
In a small bowl, combine panko, parmesan, paprika, a large drizzle of olive oil, salt and pepper. Pat chicken dry and season with salt and pepper. Place chicken on a baking sheet. Spread top side of breasts with sour cream and press panko mix into sour cream to adhere.
Toss carrots on another baking sheet with a large drizzle of olive oil, salt and pepper. Roast carrots on top rack; chicken on middle rack for 15-20 minutes.
Add couscous to boiling water - cook until tender (6-8 minutes). Drain thoroughly. Melt garlic herb butter in empty pot over medium heat. Add scallion whites and cook until softened (1 minute). Return couscous to the pot and stir until coated. Taste and season with salt and pepper. Turn off heat.
Once carrots are done roasting, remove from oven and toss with lemon zest and squeeze on lemon juice to taste.
Plate chicken, carrots and couscous; garnish with scallion greens.