Here's a hearty, flavorful dish that takes care of a few special diet needs your guests may have. It can simultaneously serve as a gluten-free, vegetarian side dish or a main course for four. Use any mushrooms you like, including cremini, oyster, or portobello. For extra flavor and vegetarian protein, garnish with chopped toasted walnuts.
Toast millet in a medium pot over medium-high heat until fragrant and just golden brown, 3 to 5 minutes. Carefully add 2 cups water, bring to a boil, then cover and simmer until liquid is almost absorbed, 20 to 25 minutes. Set aside for 5 minutes; uncover and fluff with a fork. Meanwhile, heat 1/2 tablespoon oil in a large skillet over medium-high heat. Add Brussels sprouts, onions, salt, and pepper and cook until golden, about 5 minutes. Heat remaining 1 tablespoon oil in skillet, add mushrooms; cook until golden, about 5 minutes. Add half-and-half and sage and simmer until thickened, 1 to 2 minutes more. Transfer to bowl with Brussels sprouts, add cooked millet, lemon juice, and then toss.