Nummus is traditionally fish cooked in either lemon and/or lime juice with some versions including coconut milk, but popular and super quick versions contain vinegar, here's my super quick (some may say lazy) version.

get a fillet of fresh white fish (trevally even works well with this) and finely slice it into uniform slices no more than a couple of mm thick, don't use the blood vein type flesh, just the white stuff. do the same with half a brown onion. put this into a bowl with about 1/4 cup light soy sauce, 1/4 apple cider vinegar (nicer flavour than white or brown vinegar), a few tablespoons of worcestshire sauce and a small amount of cracked pepper (optional). Add chilli to taste (as much or as little as you like), mix well and put the bowl into the fridge for a minimum of 30 minutes or overnight. If you refrigerate it overnight, give it a stir once or twice to mix the juices around the fish.

Remove the bowl from the fridge, drain the fluids and either eat as is, or serve on top of hot rice, a sprinkling of fresh coriander doesn't hurt when serving.

If you want to be "flash as" then lay the fish slices down onto a plate and arrange them into a nice pattern with the onion (possible browny points from dinner guests).

Cheers.

p.s. this is not overpowering vinegar flavours like some nummus can be, reasure the missus/friends/dog that the fish has actually been cooked by the vinegar. If you want to do a flasher version use lime juice or lemon juice (same quantities as vinegar). Add or subtract ingredients/amounts to suit your tastes, this is just a quick and dirty basic version.