Put the stock in a large, deep saucepan & turn heat to medium-high, then just barely bring it to slight boil, before turning the heat down to medium, so that it bubbles a bit.
Stir in the rice, carrot & celery & cook, stirring occasionally, until the veggies are all tender, about 20 minutes.
Season with salt & pepper & then, if you're using it, add the tomato before turning the heat down to low.
In a small bowl, use a whisk to beat the eggs with the lemon zest & juice, & then, STILL BEATING, slowly add 1/2 cup of the hot stock to the egg mixture. Continue beating while adding another cup of the stock.
Pour this mixture back into the soup & reheat, BUT UNDER NO CIRCUMSTANCES ALLOW THE MIXTURE TO BOIL OR THE EGGS WILL SCRAMBLE!
Taste & add salt, pepper or lemon juice as necessary, before serving.