Make the caramelized onions. You can do this step up to a week in advance if you like and store onions in an airtight container in the fridge.
Meanwhile, dice bacon and place in another large frying pan . Cook over medium heat until just crispy, then remove bacon from pan and set aside. Pour off most of the bacon grease from the pan, leaving two tablespoons.
To the pan with the bacon grease, add the rice and stir to coat with fat. Cook, stirring occasionally, for about two minutes, until some of the rice grains are translucent.
Raise heat to medium-high. Add the wine and cook, stirring frequently, until absorbed.
Add chicken stock, about a cup at a time, and stir frequently. Wait until most of the liquid has been absorbed and the rest turned into a starchy-looking broth before adding the next ladle-full of stock.
After the last addition of stock, continue to stir until the rice is tender, with a little chewy bite, and there is still some thick, glossy broth.
Add the spinach along with the reserved bacon and caramelized onions and and stir for less than a minute, until spinach is just wilted.
Turn off the heat and stir in the butter and the grated cheese. Season to taste with salt and pepper. Let rest for just a couple of minutes and serve in shallow bowls with extra grated cheese to pass at the table.