Cut horizontally through center of roast, cutting to, but not through, other side using a sharp knife; open flat as you would a book. Place roast between 2 sheets of plastic wrap; pound to a 1/4-inch thickness using a meat mallet or small heavy skillet.
Combine 2 cups juice and next 3 ingredients (through 2 tablespoons sugar) in large bowl; stir until sugar and salt dissolve. Pour juice mixture into a large zip-top plastic bag. Add roast to bag; seal. Marinate in refrigerator 4 hours, turning occasionally. Remove roast from bag, and pat dry; discard marinade.
Preheat oven to 350°.
Combine pear, cheese, and dried cranberries; sprinkle cheese mixture over roast, leaving a 1/2-inch margin around outside edges. Roll up roast, jelly-roll fashion, starting with long side. Secure at 2-inch intervals with twine. Sprinkle with 1/8 teaspoon salt and pepper.
Heat oil in a large nonstick skillet over medium-high heat. Add roast; cook for 5 minutes, turning to brown on all sides. Place roast on a broiler pan coated with cooking spray. Bake at 350° for 40 minutes or until a thermometer registers 160° (slightly pink). Let stand for 15 minutes before slicing; cut into 1/4-inch-thick slices.
While roast bakes, add onion to skillet; sauté 8 minutes or until onion is browned, stirring frequently. Add fresh cranberries, 1/2 cup juice, and 3 tablespoons sugar to pan, scraping pan to loosen browned bits. Reduce heat; simmer 5 minutes or until cranberries pop and sauce thickens. Serve sauce with roast.