I serve this often. I always seem to have the ingredients, so it is simple. Also, I can usually get the whole prep mess cleaned up while the soufflé cooks so it is a one-dish dinner and easy for that reason.
5 eggs separated
4 tablespoons butter
4 tablespoons flour
1 cup milk or half & half
1⁄2 teaspoon salt
1⁄2 to 1 cup grated sharp cheddar cheese
1 package frozen spinach cooked in a sauté pan until dry with a little salt, nutmeg, and butter. (Better yet, fresh spinach.)
First cook the spinach and set aside to cool. Grate the cheese.
Beat egg whites until stiff. Set aside.
In a large saucepan, melt butter, add flour, stir until bubbly, then gradually add milk or half & half, whisking until smooth. Let the sauce stand off the heat for about 3 - 4 minutes to cool a bit. Add egg yolks to the sauce. Return to heat for a couple of minutes. Be careful not to boil the sauce or the eggs will cook. Divide the sauce. (I pour half the sauce in with the spinach) Then add the cheese to the other half of the sauce. Stir the cheese into the sauce until mixed. Add half the egg whites to the cheese sauce, and the other half of the egg whites to the spinach sauce. (You are making 2 separate soufflés really.)
Fold in egg whites so as to preserve the air bubbles. No need to have a perfect looking mix. Pour the spinach mixture into a soufflé dish or other straight sided oven proof dish. Then pour in the cheese mixture on top. This soufflé isn't going to puff up a lot because of the density of the cheese and spinach so you can feel free to fill the soufflé dish close to the top.
Bake 30 minutes or so at 350º. Try not to peek as the soufflé will fall. Serve immediately.
Note: This is a basic soufflé recipe and can be varied to taste. Leave out the spinach or the cheese, try different cheeses, whatever.