Using a vegetable peeler, peel 3 long strips of lemon zest, avoiding white pith. Thinly slice zest; set aside. Squeeze 1 Tbsp. lemon juice and toss with brussels sprouts in a medium bowl.
Heat butter and oil in a large saute pan over medium heat. Saute sprouts for 2 minutes, until bright green and tender. Add salt, pepper, garlic, zest, and poppy seeds and cook an additional 2 minutes. Pour in stock and cook 1 minute more.