Heat oil in a large skillet over medium-high heat. Season both sides of steak with salt, pepper and mesquite seasoning. Sear steak in hot pan 2 minutes per side, until browned. Remove steak from pan and add sliced onions and ale to pan. Bring to a simmer, scraping up any brown bits from the bottom of pan. Return steak to pan and simmer 2 to 4 more minutes for medium doneness. Remove steak from pan (reserving sauce in pan) and let rest 10 minutes. Reduce heat to low and let ale simmer down while steak rests. Cut steak crosswise into thin slices and serve the steak, onions and pan sauce over quinoa.