Hollow out tomatoes, reserving insides. Lightly salt shells and invert on paper towel for 30 minutes to one hour to drain.
Preheat oven to 350°F. Deseed and roughly chop tomato insides. Combine orzo with olives, parsley, feta, and onions. Add lemon juice and one cup chopped tomatoes. Set hollowed tomatoes close together in a baking dish; stuff with orzo mix. Filling can spill over, or extra may be saved for another occasion. Top with pine nuts. Bake until tomatoes are cooked and hot throughout, but not falling apart, about half an hour.