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 and season with salt and pepper; arrange cut sides down on a lightly oiled baking sheet. Peel garlic; place clove 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 on top rack until bell pepper is lightly charred and garlic is softened, 20-25 minutes. Open package of chicken and drain off any excess liquid. Rinse shrimp under cold water and pat dry with paper towels. Season chicken or shrimp all over with salt and pepper. Heat a drizzle of oil in a large pan over medium-high heat. Once pan is hot, add chicken or shrimp and cook, stirring occasionally, until cooked through, 4-6 minutes. Turn off heat; transfer to a plate.

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. Keep empty pot handy for step 5.

Once bell pepper and garlic clove are done roasting, carefully 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 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 chicken or shrimp into sauce along with bell pepper and mashed garlic.

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. Chicken is fully cooked when internal temperature reaches 165°.