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. Place on a baking sheet; drizzle each half with olive oil and season with salt and pepper. Arrange cut sides down. Roast on top rack until tender, 20-25 minutes.

While bell pepper roasts, trim and thinly slice scallions, separating whites from greens. Dice tomato. Zest and quarter lemon.

Melt 1 TBSP butter (2 TBSP for 4 servings) in a large pan over medium-high heat. Add scallion whites; cook, stirring, 1 minute. Add tomato; cook, stirring, until slightly softened, 2-3 minutes. Add 1/3 cup water (1/2 cup for 4) and cream cheese. Bring to a boil, then reduce to a low simmer. Cook until sauce has thickened, 2-4 minutes. Season with salt and pepper. Turn off heat.

Once water is boiling, add tortelloni to pot. Cook, stirring occasionally, until tender and floating to the top, 3-5 minutes. Reserve 1/2 cup pasta cooking water, then drain.

Transfer roasted bell pepper to a cutting board; thinly slice. Heat pan with sauce over medium-low heat. Stir in cooked tortelloni, sour cream, half the Parmesan, 1 TBSP butter (2 TBSP for 4), and as much lemon zest and lemon juice as you like. Season with salt and pepper. Cook until cheese is melted and pasta is coated, 30-60 seconds. Stir in sliced bell pepper. If needed, stir in reserved pasta cooking water a splash at a time until tortelloni is coated in a creamy sauce.

Divide tortelloni between bowls. Sprinkle with scallion greens and remaining Parmesan. Serve with any remaining lemon wedges on the side.