Prep the broccolini by rinsing it and pulling off any leaves. Wipe the mushrooms clean and slice them longways into 3-5 thin pieces.
Add olive oil to a large saucepan over medium heat. Allow the oil to get hot first, then add the broccolini, mushrooms, salt, and pepper. Toss to coat the vegetables in the oil, salt, and pepper.
Cover and cook for about 5-7 minutes, until the juices release from the mushrooms and steams the broccolini. Then, remove cover and continue to sauté until the liquid is evaporated and broccolini is slightly charred.