Pour the buckwheat into a pan and sprinkle in a little salt. Cover with 150ml boiling water and pop on a lid. Return to the boil, then reduce the heat to a simmer and cook for 8-10 mins. Once cooked, drain the buckwheat and rinse with cold water.
Meanwhile, peel the celeriac. Scrub the beetroot and trim any stalky ends away. Quarter the apple, and slice out the core. Use a box grater to coarsely grate all three (or use the grating attachment on your food processor to whizz the veg) and slide them into a large bowl.
Place a pan on a medium-high heat and warm for 2 mins. Scatter the cashews into the dry pan and toast for 3-4 mins, tossing frequently, till golden brown on the outside. Tip into a small bowl.
Peel the garlic and crush it. Finely grate the lemon zest. Slide both the garlic and zest into a bowl. Spoon in 1 tsp horseradish mustard. Squeeze in the lemon juice and pour in 1 tbsp oil. Sprinkle in a little salt and pepper and stir well. Have a taste, and add more horseradish mustard, if preferred.
Shake the buckwheat dry and add to the bowl with the grated veg. Tip in the spinach and toasted cashews. Spoon in the dressing and fold together to mix. Pile the salad onto plates and serve.