Start with peeling the onion and slicing it lengthwise to give nice long strips.

Slice the pepper, any color will do, and remove the seed pod.  I used to do this with a knife and found that it took more time and involved more risk than snapping the seed pod out with my fingers.

Rinse the pepper inside and out once the seed pod is removed.
Then slice the pepper into long thin strips.

Add onions and peppers to cast-iron skillet.  Then add some oil which will handle the high heat.  
This is one dish where I feel both the cast-iron and a high-heat oil are necessary.  I like to cook
my fajitas at a very high heat so I make sure to use peanut oil which can handle the high temperature without smoking.  I only use about 1 or 2 tablespoons of oil.

A quick comment on cast-iron skillets.  There is nothing like a cast iron skillet. It is the original non-stick cookware and a must-have for kitchens. It handles very high heats and cooks and browns at the same time.  (A good cast iron skillet is also GREAT for cooking cornbread in the oven.  Nothing else makes the cornbread moist on the inside and "crusty" on the outside.)  

Time to remove the peppers and onions and get them ready for serving. If I wasn't going to make some portabello mushrooms, I would serve these in the cast iron skillet. Serving them at the table from the skillet keeps them warm longer and looks so authentic! But tonight, a nice bowl will have to do!

Now for the mushrooms!  Start with some portabello mushrooms which are nice and firm and not slimy.  (If yours have gone a little limp, it's okay to still use them.  In this dish, you won't even notice!)  I give my mushrooms a quick rinse under running water and shake off the excess.  True mushroom gourmets will probably tell you not to do this - but it works for me.  Taking the time to gently wash the mushrooms or rub them - well, that just takes too long!  Since I am going to be frying these up, I don't bother to remove the "black gills" underneath the cap.  Some recipes will call for this, however, unless you are really concerned about the appearance, don't bother.  Make sure to slice the mushrooms nice and thin. I like to cut mine to approximately the same size as the peppers and onions.

Place the portabellos in the cast iron pan you just used for the peppers and onions.  Sometimes I will
add a little more oil, but they generally don't need it. The little bit of water left after washing the mushrooms is enought to get them started.  Once the portabellos begin to heat up, they will release quite a bit of  water and they won't stick to the pan.  Usually  I cover the pan with a domed lid for a few minutes to help hold the heat in and get the portabellos nice and hot quickly.  Stir them a few times while cooking and remove them before they get too limp.