Heat 1 tablespoon oil in a frypan and cook the onion until soft. Lower heat and cook onion about 10 more minutes until it's starting to get crunchy.
Brush the sourdough slices with oil and toast a few minutes either side under the grill, until just browning.
Heat a frypan with remaining oil and sear the steak slices about 30 seconds a side, seasoning with salt and pepper. Set aside. Add the spinach to the frypan to wilt.
Top a slice of sourdough with the spinach, then two slices of steak, then half the onions. Add chutney if desired and top with another slice. Repeat with remaining ingredients.