Heat a cast iron pan over medium-high heat. Add the butter and cook until it foams. Add the leeks and sautee, stirring occasionally, until soft and slightly browned along the edges, 3-4 minutes.
Push the leeks aside and add the brussels sprouts.Sautee, stirring occasionally, until the sprouts are caramelized, 7-8 minutes.
Add the spinach to the pan and sautee, stirring continuously, until the spinach is barely wilted, about 1 minute.
Remove the pan from heat. Sprinkle the vegetables with salt and pepper to taste and the parmesan cheese if using. Crack the eggs over the vegetables, taking care to keep the yolks intact. I find it useful to make a little depression in the vegetables with a spoon to dump the whites into, followed by the yolk.
Transfer the pan to the oven and cook until the whites are set, but the yolks are still runny, 8-12 minutes. Serve immediately.
To prepare this dish in advance, cook the vegetables, excluding the spinach. Divide into single or double portions (or whatever size makes sense for you). Then, dice up your spinach, divide into portions, and roll each portion in a paper towel; place the towel-wrapped spinach into a zip lock bag and then stick everything in the fridge. The cooked vegetables will keep well for about four days, the spinach a week or more. The day you'd like to have this dish, preheat the oven to 435 and heat up a skillet over medium-high heat; add just a tiny bit of butter (maybe a teaspoon) until it foams; add the brussels and leeks and sautee 1 minute.The day you'd like to have this dish, preheat the oven to 425 and heat up a skillet over medium-high heat; add just a tiny bit of butter (maybe a teaspoon) until it foams; add the brussels and leeks and sautee 1 minute; add the spinach and sautee until wilted. Sprinkle it all with salt, pepper, and parmesan, crack an egg or two (or 4) over the top, and stick it in the oven for 8-12 minutes. Then go have a shower. Or scroll through Facebook. Or make the kids' lunches. Or whatever. You get the point.