Add extra-virgin olive oil, 2 turns of the pan in a slow stream.
Add sliced mushrooms and let them brown.
While the mushrooms begin to cook, cut the peppers in half and pull out the seeds and connective membranes and throw them out. Cut peppers in quarters lengthwise, then slice across.
Cut onion in half and peel it. Cut off ends and cut the half onion in half lengthwise, then slice across.
Add peppers and mushrooms and season with salt and pepper. Cook for another 3-5 minutes, stirring often, until all veggies are just tender but still have a little crunch left to them and lots of bright color.
Remove veggies from the heat and add 2 Tablespoons of cilantro-lime butter.
Toss veggies to melt the 2 Tablespoons of butter and then serve.
Cut butter into chunks. Place butter in bowl. Place bowl in microwave and cook on high for 15 seconds.
Stir in cilantro and lime zest and juice into the butter using a rubber spatula.
Transfer the butter onto a large strip of plastic warp.
Gather the ends of plastic wrap in 1 hand and use a straight edge, like a small cookie sheet pan, to push the butter back away from your body making the butter take on a log shape. Gently roll the log wrapped in plastic back and forth on the countertop to evenly round out the shape.
Twist up the extra wrap on the ends and place the compound butter log into the freezer to amke the butter cold enough to slice again.