Puree the roasted peppers together with the liquid from the jar in a smoothie cup, blender, food processor, or with immersion blender, until smooth.
In a large skillet, saute the onion in olive oil over medium heat, until soft (about 7-10 minutes).
Add garlic, cook for 1 minute and then add mushrooms. Cook, stirring occasionally, for 10 minutes.
Add pureed roasted peppers, salt, pepper, Italian herbs, and red pepper flakes. Stir.
Add calamari and cook them in the mushroom-pepper sauce for 20-30 minutes, until calamari are tender.
While calamari are cooking, bring a large pot of water to a boil and cook the fettuccine according to the package instructions. Drain, rinse with cold water, and set aside.
Add Half&Half to calamari sauce, stir and turn off the heat.
Add fettuccine to the skillet and mix everything well. Garnish with chopped parsley.