Using the whisk on a hand mixer, beat the butter for approx. 2 mins. until light and creamy. Stir in the onion and all the other ingredients up to and including the chilli, season with salt. Place the butter on a piece of baking paper, shape into a roll, refrigerate for approx. 30 mins.
Mix the asparagus with the oil in an aluminium grill tray.
Charcoal/gas/electric grill: Brush the steaks with oil, cover and grill over/on a very high heat (approx. 250 °C for approx. 1½ mins. on each side, season. Cover the steaks and leave to rest for approx. 5 mins. Grill the asparagus over/on a very high heat for approx. 5 mins., turning occasionally, season at the end. Carve the steak into thin slices, plate up with the asparagus. Serve with the chimichurri butter.