You will need
Two large eggs
About one ounce or one tablespoon of butter
And the same quantity of flour.
Half a cup of milk (I use skim). If you're counting calories, or carbs, or just like green, you could use the water you cook the spinach in.
1 tsp of dijon mustard
a pinch of paprika, or maybe cayenne
Some spinach, cooked (whatever will make about 1/2 cup when cooked)
Cheese - to make about 1/2 cup grated. I use aged cheddar, but feta or blue cheese would work well, or some combination, as long as it's fairly piquant.
Some kind of oven-proof ceramic or glass receptacle big enough to hold all above ingredients mixed together with room for rising. I used a china pudding basin (the kind you make Christmas puddings in) because my souffle dish is too big. This should be greased.
Pre-heat oven to 450 degrees.
Cook spinach, and drain, reserving the water if you want to use it.
In a small saucepan, melt the butter, add the flour and the milk or spinach-water to make a fairly stiff batter rather than a sauce. Add the cheese until it melts, the mustard, and the paprika or cayenne. Set aside.
Separate the eggs. Whisk the egg yolks until lemon coloured. Beat the whites until peaks form. They should be stiff but not dry; you are not making meringues here.
In a medium bowl, combine the cheese sauce and the egg yolks. Mix lightly: the idea is to keep everything airy, so don't beat the hell out of it. Add the cooked spinach.
Now the tricky part: with a _spoon_, add about one-third of the beaten egg whites and mix lightly. This is what Julia Child called "lightening the batter." Then, with a spatula, fold in the remaining egg whites. Plop the whole mixture into your greased receptacle. Again, it's important to keep everything light - you want air bubbles, so don't mix things too much.
Put in the oven and immediately turn the oven down to 400 degrees. This is a clever way of keeping the temperature constant. Bake for about 30 - 40 minutes, depending on the depth and thickness of your receptacle, until the souffle is risen and is brown and smells done.
If you have a friend over, double the quantities, follow the same instructions, but bake for an hour.
Serve immediately, with a green salad, or some steamed asparagus, or something else delicious, and some white wine.