Omelette w/ portabella mushrooms, red onion and medium cheddar cheese

3 eggs, well beaten with a splash of milk
Handful chopped baby portabella mushrooms
Handful finely chopped red onion
4 slices medium cheddar cheese**

Over medium heat, saute onion and mushrooms in a bit of butter and olive oil until soft -- 3 or 4 minutes.  (I read somewhere recently that adding olive oil keeps the butter from burning.  Very handy tip.)  Add a little pepper if you like.  Salt is not necessary.  There is plenty of salt in the butter and cheese.  Scrape into small bowl and set aside.  Return pan to heat.  Add a little more butter.  (You can use non-stick spray if butter is not your friend.)  When butter has melted, pour in the beaten eggs.  Cook, without flipping, until eggs are almost set.  Spread onions and mushrooms over one half of the eggs.  Layer cheese over mushrooms and onions.  Fold eggs up over mushroom, onion, cheese mixture so that you have a sort of half moon of egg with the goodies in the middle.  Lower heat slightly to prevent burning.  Once you think that everything will hold together, gently flip the omelette.  Use a spatula rather than the courage of your convictions.  After another minute, turn off heat.  Serve omelette on warmed plate.  (It may seem like a minor detail, but a warmed plate makes a big difference.  Run it under warm water.  Set it on the stove while cooking if it won't be in the way.  Or pop it in the microwave for 30 seconds or so.)

**Swiss would be better, but cheddar is what I had in the fridge.