I am not at all sure this is the correct way to cook oeufs florentine. In fact, it probably isn't - but who cares, because it's very quick and very easy. I had planned to take a photograph when I had served it, but it flipped over and all you could see was an unpretty mound of spinach!
I never used to like spinach when I was a child, but I think that, rather like kale, modern cultivars are milder in taste. But even though it must be fifteen years or so since I discovered that actually, it was edible, I was still a bit wary of it as it seemed to have a taste that overwhelmed everything else on the plate, rather like coriander can if you use too much. But the baby spinach, sold for use in salads, is also very mild when cooked - yes, it tastes like spinach, of course, and if you don't like that you won't like it, but it doesn't dominate or overpower. My brother, who has always liked spinach, says he just snips the corner of the bag and shoves the lot in the microwave! But don't do that if you aren't sure that the spinach has been washed, or it's a good way to get a mouthful of grit! And, of course, it is also lovely raw, in a salad or sandwich.
1 dollop of butter (about 20g - what Americans would call a teaspoonful, I suppose)
1 large handful baby spinach, washed if necessary
Salt and pepper to taste
Melt the butter in a small saucepan - a milk saucepan is ideal - and put the spinach on the top. Allow it to cook for a minute or two, and stir vigorously to make sure all of it wilts and cooks. Now lower the heat and break and egg on the top, leaving this to cook (cover the pan, if possible) for several minutes until the white is firm and the yolk as firm as you like it.
Serve on (ideally) a toasted muffin or a slice of toast. Or not, if you're being low-carb and can digest an egg without bread....