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 stem and seeds. 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 and garlic have roasted 15 minutes, add agnolotti to pot of boiling water; reduce to a low 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.

Once bell pepper and garlic cloves are done roasting, transfer to a cutting board. Thinly slice bell pepper and gently mash garlic with a fork until smooth.

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

Stir agnolotti into sauce. (TIP: If needed, add more reserved pasta cooking water a splash at a time until pasta is fully coated.) Divide between bowls. Season with pepper. Garnish with scallion greens and remaining Parmesan and serve.