Dice the bacon and fry it in a large skillet until crisp. Remove with a slotted spoon and drain on paper towels. Pour the bacon fat into a glass, heat-proof, measuring pitcher. Measure 4 TBLS, and put it back into the skillet - discard the remainder. Over low heat, add sugar and vinegar to the pan, and stir until dissolved. Add the onion sliced and cook, stirring, until the onion is softened. Pour the hot dressing over the spinach and toss with tongs until the leaves are well-coated and slightly wilted. Salt, and grind in black pepper to taste, and toss again. (If you want the spinach to cook more, place it in a metal bowl and put the bowl on the warm burner where you heated the dressing while you toss the salad.) Divide the salad onto 4 plates. Slice an egg on top of each salad, and add about 1/4 of the pears cubes to each plate. Top with the bacon bits, and cheese, if desired, and serve right away.