Drain the jar of roasted peppers into a blender or food processor, include the jar liquid, and puree until smooth; set aside.
Heat the olive oil in a large skillet over medium heat, add the diced onion and cook until soft, about 7–10 minutes.
Add the minced garlic and cook 1 minute, then add the sliced mushrooms and sauté, stirring occasionally, for about 10 minutes.
Stir in the pureed roasted peppers, salt, black pepper, dried Italian herbs, and red pepper flakes; combine well.
Add the calamari rings to the skillet, reduce heat to maintain a gentle simmer, and cook 20–30 minutes until the calamari are tender.
While the calamari cooks, bring a large pot of salted water to a boil and cook the fettuccine according to package instructions; drain and briefly rinse with cold water, then set aside.
Stir the Half & Half (or heavy cream) into the calamari and mushroom sauce, then remove the skillet from the heat.
Add the cooked fettuccine to the skillet and toss gently to coat the pasta in the sauce; adjust seasoning with salt and pepper if needed.
Serve garnished with chopped parsley.