Adjust rack to top position and preheat oven to 400 degrees. Bring a medium pot of salted water to a boil. Wash and dry all produce. Halve bell pepper; remove seeds and stems. Trim and thinly slice scallions, separating whites from greens.

Drizzle bell pepper halves with olive oil, salt, and pepper; arrange cut sides down on a lightly oiled baking sheet. Place garlic cloves in the center of a small piece of foil; drizzle with oil and season with salt and pepper. Cinch into a packet and place on same sheet. Roast until bell pepper is lightly charred and garlic is softened, 20-25 minutes.

Once bell pepper has roasted for 15 minutes, add agnolotti to pot of boiling water; reduce to a simmer. Cook until al dente and floating to the top, 3-5 minutes. Reserve 1/2 cup pasta cooking water (1 cup for 4 servings), then drain and set aside.

Transfer roasted bell pepper and garlic cloves to a cutting board. Thinly slice bell pepper and gently mash garlic with a fork.

Heat a drizzle of olive oil in pot used for agnolotti over medium-high heat. Add scallion whites and cook until fragrant, 30 seconds. Add 1/4 cup reserved pasta cooking water (1/3 cup for 4 servings), stock concentrate, cream cheese, sour cream, half the Parmesan, and 1 TBSP butter (2 TBSP 4); stir until smooth. Stir in sliced bell pepper and mashed garlic. Bring to a simmer and cook until sauce is slightly thickened, 1-2 minutes. Turn off heat. Season with salt and pepper.

Stir agnolotti into sauce. If sauce seems dry, add a splash or two more of reserved pasta cooking water. Divide between bowls. Garnish with scallion greens, pepper, and remaining Parmesan.