Preheat the oven to 450 degrees F. Toss the sweet potatoes, red onion, 1 tablespoon olive oil, 1/4 teaspoon salt and a few grinds of pepper on a rimmed baking sheet; arrange the onion cut-side down. Roast, flipping halfway through, until the sweet potatoes are tender and the onion is browned and crisp, 25 to 30 minutes.
Meanwhile, season the steaks with salt and pepper. Heat a large skillet over medium-high heat and add the remaining 2 tablespoons olive oil. Add the steaks and cook until browned, 3 to 4 minutes per side for medium rare. Transfer to a plate and tent with foil; let rest.
Remove the skillet from the heat and add the beer, scraping up any browned bits from the bottom of the pan. Stir in the mustard. Return the skillet to medium-low heat and simmer until the liquid is slightly syrupy, 1 to 2 minutes. Add the vinegar, honey and any collected juices from the steaks; season with 1/4 teaspoon salt. Remove from the heat and stir in the butter until melted.
Peel and slice the roasted onion. Top the steaks with the pan sauce. Serve with the roasted vegetables. Sprinkle with the parsley.