Prepare the Brussels sprouts by trimming the ends and removing any yellowing outer leaves. If sprouts are on the larger side, halve lengthwise. Set aside.
In a medium bowl, stir together butter and rosemary until thoroughly combined. Season to taste with pepper. Melt half of the compound butter in a large skillet over medium-high heat. Add the Brussels sprouts to the pan and toss to coat. Reduce heat to medium and cook, about 8 minutes, until crisp-tender and beginning to brown in places.
Meanwhile, whisk together miso and 1 T. lemon juice in a small bowl. Heat briefly in microwave, about 15 seconds, and whisk until thoroughly combined and mixture reaches a sauce-like consistency, thinning with additional lemon juice or water if necessary. After Brussels sprouts are cooked, add remainder of compound butter to pan and allow to melt into the Brussels sprouts, about 1 minute more. Add miso-lemon sauce to pan and toss to coat.
Transfer to a serving dish, garnish with lemon zest, and serve.