For filling, in a food processor combine the first five ingredients (through garlic). Cover and process with several on and off turns until smooth. Stir in the green onions. Transfer to a resealable plastic bag.
Cut a T-shape slit in one side of each sweet pepper starting horizontally under the stem and then cutting vertically from the center of that cut to the tip of the pepper. Carefully remove and discard seeds, leaving stems intact. Snip one corner of the plastic bag with the filling; carefully pipe filling into peppers. (You may have extra filling.)
For sauce, in a medium bowl combine the next four ingredients (through pepperoncini peppers). Pour half of the sauce into a 6-quart slow cooker. Top with stuffed peppers. Carefully add remaining sauce.
Cover and cook on low for 4 hours or on high for 2 hours. Serve immediately or keep warm, covered, on warm or low for up to 2 hours. If desired, sprinkle with Parmesan cheese and/or parsley.