In a small saucepan combine the port, cherries, and sugar. Cook and stir until boiling, stirring to dissolve sugar. Reduce heat and simmer, uncovered, for 10 minutes. Remove from heat. Cool completely (or, cover and chill overnight). Drain cherries, reserving port.
Preheat oven to 400 degrees F. In a large skillet cook sausage over medium heat until browned, stirring to break up pieces. Remove sausage from skillet with a slotted spoon, reserving drippings. Add butter to skillet. Add onion; cook and stir until tender, about 5 minutes. Gradually add spinach, cooking and stirring until wilted, about 2 minutes. Remove to a colander set over a large bowl. Press out liquid with the back of a spoon; discard liquid. Coarsely snip spinach mixture. In the bowl stir together the spinach mixture, drained cherries, cheese, bread crumbs, and parsley. Set mixture aside.
Pound chicken breasts between 2 pieces of plastic wrap to an even 1/2-inch thickness; set aside. On 2 pieces of plastic wrap overlapped together lay 6 18-inch pieces 100% cotton kitchen string vertically at about 2-inch intervals. Weave a 24-inch piece of string across the strings. Lay bacon strips at a slight diagonal across the strings, overlapping slices. Arrange chicken breasts evenly on top of bacon, leaving a 1 1/2-inch bacon border at the top and bottom and overlapping the chicken pieces slightly. Press to make even thickness. Sprinkle with salt and pepper. Spread sausage mixture across center of chicken. Quickly lift and fold chicken and bacon over filling, using plastic wrap to lift. Tuck chicken and bacon under opposite side to form a roll. Using plastic wrap, turn seam side up. Pull plastic wrap away and tie strings to make a tight roll, tying the string that crosses the roast lengthwise last. Add more string as necessary to secure.
Line a roasting pan with foil; add a rack. Transfer roast, seam side down, to rack in prepared pan. Roast for 1 3/4 hours or until done in center (165 degrees F). Remove and let stand 10 minutes before slicing. Serve with remaining port sauce.