Put the spinach in a sieve and rinse under cold water to thaw it. Squeeze to remove all the liquid—then squeeze some more; you want it to be very dry and crumbly. Put the spinach on a cutting board and use a sharp knife to finely chop it. Transfer to a bowl and stir in the labneh, salt, and cumin, if using, with a fork, breaking up the clumps of spinach as you go. Serve, or cover and refrigerate until evening.