Peel and finely chop the red onion. Tip it into a small bowl. Halve the cucumber. Finely dice one half (see our tip on what to do with the remaining half). Add it to the red onion.
Pick the leaves off the mint sprigs. Finely chop the leaves and add them to the chopped veg.
Put a dry frying pan on a medium heat. Add 1 tsp cumin seeds. Toast for 1-2 mins till they smell aromatic and start to pop. Add to the bowl.
Add the black mustard seeds to the pan and toast them for 1-2 mins. Add to the bowl. Add 1 tbsp olive oil and a pinch of salt and pepper to the bowl and stir to mix.
Slice the beef steak tomatoes into think rounds. Arrange them on two serving plates. Spoon the red onion and cucumber mix over the top. Top with spoonfuls of the cheese and serve.
Your remaining cucumber half will keep for a few days in the fridge. Try grating it, then mixing it with yogurt, crushed garlic and chopped mint as a delicious dip for crudites.