Skirt Steak with Balsamic-Glazed Onions and Creamy Polenta

Preheat oven to 425 degrees F. Arrange broccoli on a baking sheet and drizzle with 2 tablespoons olive oil. Season with salt and pepper. Roast until golden around edges, 15 to 20 minutes. Meanwhile, in a medium saucepan, bring 4 1/2 cups salted water to a boil. Add polenta in a slow, steady stream, whisking constantly. Bring to a boil, then reduce heat and let simmer on low, whisking regularly, until thickened, 20 minutes. Stir in butter and season with salt and pepper. In a small saucepan over medium-high heat, heat 1 tablespoon olive oil. Cook onion until beginning to soften, 3 minutes. Stir in brown sugar and cook 2 minutes more. Add balsamic vinegar and bring to a boil, then reduce heat and let simmer until texture resembles a thick syrup, about 5 minutes. Season with salt and pepper. Heat a grill pan over high heat. Season skirt steak with salt and pepper. Grill 3 to 4 minutes per side for medium rare. Let rest 5 minutes and cut thinly on the diagonal. Serve steak over polenta topped with balsamic red onions. Serve broccoli on the side.