Place pears in a bowl; drizzle with lemon juice. Fill bowl with enough water to cover pears. Place a small plate over pears to submerge pears; set aside. In a large bowl combine spinach and red onion. If desired, sprinkle with pepper; set aside.
For dressing, in a Dutch oven cook bacon until crisp. Remove bacon, reserving 1/4 cup drippings in Dutch oven. (If necessary, add enough oil to equal 1/4 cup). Crumble bacon; set aside. Stir the 1/2 cup cranberries, the vinegar, sugar, and mustard into drippings in Dutch oven. Bring to boiling; remove from heat. Add spinach mixture. Toss mixture in skillet for 30 to 60 seconds or just until spinach is wilted.
Transfer spinach mixture to a large bowl. Add bacon; toss to combine. Divide spinach mixture among six salad plates; drain pears and arrange on top of spinach mixture. If desired, sprinkle with Gorgonzola cheese and additional cranberries.