Cook the pasta until tender; drain and rinse under cold water, toss with 1-2 Tbsp oil so the pasta won't stick together; set aside.
Meanwhile, for the sauce, in a heavy-bottomed small saucepan, stir the cream cheese and 1/4 cup half and half over med-low heat until smooth.
Stir in remaining half and half cream.
Stir in the sweet peppers OR pimiento (if using), and Parmesan cheese; heat through, and mix.
For the filling, in a large bowl, stir together 3/4 cup of the sauce (set the remaining sauce aside), the cooked chicken, broccoli, onion, salt and pepper.
Using a small spoon, stuff each manicotti with about 1/4 cup of the filling.
Place in the prepared baking dish.
Bake, covered for 20-30 minutes, or until heated through.
Place 2 manicotti on each serving plate, spoon sauce over the shells.
NOTE:If desired, fresh minced garlic may be added to the cream sauce.