Place the steak, garlic, onion and cheese into a food processor and blend until combined, but not smooth.
Fill the up-turned middle of the mushroom with the mixture and sprinkle with the sage.
Heat an ovenproof pan with the oil and place the mushroom filling-side up into the hot pan. Cook for one minute, then place into the oven to bake for 10 minutes, or until the filling is golden and completely cooked through.
To serve, place the mushroom on a plate and top with the chives.
