Halve the Brussels sprouts lengthwise, keeping the stem end intact so they hold together during cooking. Slice the mushrooms into roughly 1/4-inch thick pieces. Dice the onion into 1/2-inch pieces for even cooking. Slice the kielbasa into 1/4-inch rounds and mince the garlic. Having everything prepped before you start cooking ensures the dish comes together smoothly without scrambling mid-recipe.

Heat the olive oil in a large skillet over medium-high heat until shimmering. Add the sliced kielbasa and cook for 3-4 minutes, stirring occasionally, until the edges are lightly browned and some of the fat renders out—this creates a flavorful base. Add the diced onion and minced garlic, stirring frequently for another 4 minutes until the onion softens and becomes translucent. I like to cook the kielbasa and aromatics separately from the vegetables first because it allows the onion to caramelize slightly and the garlic to bloom, creating much deeper flavor than if everything cooked together.

Add the halved Brussels sprouts and sliced mushrooms to the skillet, stirring well to combine with the kielbasa mixture. Cook uncovered for 8-10 minutes, stirring occasionally, until the Brussels sprouts are tender and develop golden-brown edges on their flat sides, and the mushrooms have released their moisture and begun to brown. The Brussels sprouts will have a slight give when pierced with a fork when ready.

Taste the dish and season generously with salt and freshly ground black pepper to your preference. I always recommend freshly ground pepper here because it adds a subtle spiciness that complements the richness of the kielbasa and earthiness of the mushrooms. Serve immediately while everything is still hot.