Preheat the oven to 425°F (220°C). Trim the woody ends from the brussels sprouts and halve any larger sprouts so all pieces are roughly the same size (about 1/2–3/4 inch). Wash and thoroughly dry the brussels sprouts and torn kale leaves. Zest the lemon, then cut and reserve juice.

Roast Brussel Sprouts

Toss the halved brussels sprouts with 1 tablespoon olive oil, 3/4 teaspoon salt, 1/2 teaspoon black pepper, and 1/2 teaspoon red pepper flakes on a rimmed baking sheet so they form a single layer with cut sides down. Roast in the preheated oven for 18–22 minutes, until the cut sides are deep golden-brown and outer leaves are crisp. Stir gently once at the 12-minute mark if needed for even browning.

Sauté Kale & Garlic

While the brussels sprouts roast (about 6–8 minutes before they finish), heat a large skillet over medium heat. Add 1 tablespoon olive oil and 1 tablespoon butter. When the butter is melted and foaming, add the minced garlic and sauté 30–45 seconds until fragrant but not browned. Add the torn kale, the remaining 1/4 teaspoon salt, remaining 1/2 teaspoon black pepper, and remaining 1/2 teaspoon red pepper flakes. Sauté, tossing frequently, until the kale is wilted and tender, about 4–6 minutes. If the pan gets dry, add 1–2 tablespoons water to help steam.

Combine & Finish

When the brussels sprouts are done, transfer them to the skillet with the sautéed kale (or combine both in a large bowl). Add the lemon zest, 1 tablespoon fresh lemon juice, and toss gently to combine. Taste and adjust seasoning with additional salt or lemon juice as desired. If using, sprinkle the grated Parmesan over the vegetables and toss once more so it melts slightly from the residual heat.

Serve warm as a side dish. Leftovers keep in an airtight container in the refrigerator for up to 3 days; reheat gently in a skillet to keep the kale from becoming limp.