Orecchiette with Vegan Sausage and Brussels Sprouts

Name: Orecchiette with Vegan Sausage and Brussels Sprouts
Ingredients: Kosher salt and freshly ground black pepper; 2 tablespoons butter substitute, such as Earth Balance Buttery Spread; 2 tablespoons olive oil, plus more, for drizzling; 1 yellow onion, halved and thinly sliced; 6 ounces vegan Italian-style sausage, such as Fields, crumbled by hand or chopped; 1 teaspoon fennel seeds; 12 ounces dried orecchiette; 1 pound Brussels sprouts, halved lengthwise, cored and separated into leaves; 3 cloves garlic, minced; Pinch of red pepper flakes; Zest of 1 lemon