Place the lardons or diced smoked bacon in a cold heavy-bottomed pot. No oil. Bring the heat up to medium-low and render the bacon slowly for 8 to 10 minutes until the fat has run and the pieces are beginning to colour at the edges. Do not drain the fat. It is your entire flavour base.
Add the diced onion to the rendered bacon and its fat. Cook over medium heat, stirring occasionally, for 5 to 6 minutes until the onion is completely soft and lightly golden. The onion absorbs the smoky bacon fat and will perfume the entire soup.
Add the cold water directly to the pot with the bacon, fat and onion. Bring slowly to a simmer over medium heat. Add the white rice and stir to combine. Season with salt and black pepper, remembering that the smoked bacon is already salty. Simmer gently for 18 to 20 minutes, stirring occasionally, until the rice is completely cooked and has thickened the broth slightly. The finished soup should be somewhere between a thick broth and a stew, exactly the register that French military cooks were aiming for when they made their version of la soupe in the field.
Taste and adjust salt. Scatter fresh parsley over the top if you have it.