Heat your oven to 190°C/Fan 170°C/Gas 5. Put a medium pan full of water on to boil.
Peel the nubbly skin off the short cucumber. Coarsely grate the cucumber flesh and pop it into a sieve set over a bowl. Add a pinch of sea salt and then use your hands to squeeze and mix the cucumber till you’ve squeezed out as much liquid as you can. Leave the cucumber in the sieve, lay a saucer over the top of it and weigh the saucer down with a tin or 2.
Strip the leaves off the cauliflower, then slice a sliver off the base so it sits up straight on your board. Slice roughly 2cm off the side (don’t worry if these bits crumble into florets), then slice the chunky middle bit into 2 thick steaks. Set aside.
Peel and crush the garlic. Pop it in a small bowl. Stir in 1 tsp each ground cumin and smoked paprika, 1 tbsp olive oil and a pinch of salt and pepper. Rub this spice paste all over the cauliflower steaks, and dab any leftover of the broken side bits of the cauliflower (you’re going to cook it all). Peel and finely slice the red onion.
Put a large frying pan on a high heat to warm for 2 mins. Add the cauliflower steaks (no need for oil) and fry for 2 mins on each side so they brown a little, then transfer them to a baking tray. Fry the side florets for 2 mins and add to the tray too. Slide the baking tray into the oven and roast for 10-15 mins till the cauliflower is tender when you insert the tip of a knife.
The water should be boiling by now, so tip in the buckwheat. Allow to simmer for 15 mins till the grains are tender.
While the cauliflower and buckwheat cook, add the onion to the frying pan with 2 tbsp water and a pinch of salt and pepper. Fry over a mediumlow heat for 5-6 mins, stirring now and then, till softened and a little caramelised.
While the onion cooks, finely chop the mint leaves. Give the cucumber another squeeze to get rid of any remaining liquid, then transfer to a clean bowl. Stir in most of the mint and all the yogurt. This is your tzatziki.
When the buckwheat is tender, drain it and let it steam dry in the colander for 2 mins. Arrange the cauliflower steaks on 2 warm plates and spoon the onion over the top. Add a few spoonfuls of the buckwheat to each plate, then serve with the tzatziki and garnish with the last of the mint.