Preheat oven to 400°F. Toss carrots with 2 Tbsp. oil, 1 tsp. salt, and 1/2 tsp. pepper on a rimmed baking sheet. Roast, tossing occasionally, until carrots are golden brown and tender, 25–35 minutes. Let cool. Meanwhile, pulse 1/4 cup pistachios in a food processor until a coarse paste forms. Add carrot tops and 2 cups basil. Pulse until a coarse purée forms. Add remaining 1/2 cup oil and pulse until combined; season with remaining 1/2 tsp. salt and 1/2 tsp. pepper. Transfer carrots to a platter. Drizzle with pesto, then top with basil leaves and remaining 1/4 cup pistachios.