Cut the red bell pepper in half and remove seeds and stem. Place cut-side down in a baking dish. Lightly coat with oil, season with salt and black pepper, and roast for 20–25 minutes until softened and lightly charred.
Meanwhile, cook pasta according to package directions. Drain and set aside.
In a large skillet, heat olive oil over medium heat. Add garlic and cook for about 30 seconds until fragrant.
Add marinara sauce and ⅔ cup water. Stir in half of the green onions, salt, and crushed red pepper. Bring to a boil, then reduce heat and simmer for 3–5 minutes.
Turn off heat and stir in cream cheese until fully melted and smooth.
Slice the roasted red pepper into thin strips and add to the sauce along with the cooked pasta. Toss to combine.
Transfer to the same baking dish and top with mozzarella.
Broil for 4–5 minutes until the cheese is melted and lightly golden.
Sprinkle with the remaining green onions. Serve warm. Bon appétit.