Heat a large skillet over medium high heat. 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 into your garbage bowl. 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-again lengthwise, then slice the onions across. Add peppers and onions to mushrooms and season with salt and pepper. Cook veggies another 3 to 5 minutes, stirring often, until the 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 butter then serve.
Cut butter into chunks. Place butter in a bowl. Place bowl in microwave and cook on high for 15 seconds. To chop herbs super-fine, hold the tip of your knife on a cutting board and move the back of your knife up and down very fast going over the herbs again and again. Stir in chopped cilantro and lime zest and juice into the butter using a rubber spatula. Transfer the butter onto a large strip of plastic wrap.
Gather the ends of the plastic wrap in on 1 hand and use a straight edge, like a small cookie sheet pan, to push the butter back and 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 make the butter cold enough to slice again.