If you don't happen to have a hard-boiled egg in your fridge ... just cook an egg & cool it properly afterwards.
If you don't happen to have some leftovers of fried fish fillets ... just dice the fish fillet & fry it for some minutes in olive oil. Add salt & pepper to taste. Afterwards mash the fish fillet w/ a fork. If dealing w/ leftovers, just mash the fried fish fillet.
Chop a spring onion & add to the mashed fish fillet.
Whisk the Greek yoghurt w/ the oils, add salt & pepper to taste & add a chopped hard-boiled egg.
Mix about ⅔ of the dressing w/ the fish mess.
Chop the lettuce leaves, the cucumber slices, the tomatoes & the radishes.
Toast the toast - it shall be soft & crispy.
Divide the fish mess between 2 slices of toast.
Divide the rest of the dressing between 2 slices of toast.
Add the salad mix onto the fish mess & add the slice of toast w/ the dressing on top.
Press slightly together & serve at once.