Combine first 5 ingredients in a large zip-top bag, and marinate in the refrigerator overnight.
Place beef and marinade in an electric slow cooker; cook on LOW 8 hours or until beef is tender. Place beef on a cutting board (reserve cooking liquid); let stand 10 minutes. Thinly slice beef; place in a shallow dish. Pour cooking liquid over beef.
Heat oil in a large nonstick skillet over medium-high heat. Add bell pepper to pan; sauté 5 minutes or until tender. Slice rolls lengthwise, cutting to, but not through, other side. Hollow out top and bottom halves of rolls, leaving a 3/4-inch-thick shell; reserve the torn bread for another use. Arrange about 3 ounces beef and 2 tablespoons bell peppers on each roll. Drizzle 1 tablespoon cooking liquid over beef and peppers; top with giardiniera, if desired. Serve with remaining 2 1/2 cups cooking liquid for dipping.
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