Heat your oven to 180°C/Fan 160°C/Gas 4. Peel and finely slice 1 garlic clove. Pull the leaves from the rosemary. Using a small sharp knife, make small holes in the pork fillet. Push a slice of garlic and a couple of rosemary leaves into each incision. Rub with 1 tsp olive oil.
Peel and thickly slice your onions. Cut the potatoes into halves or quarters so they’re the same size.
Peel and chop the beetroot into small chunks, smaller than the potatoes. Halve the lemon lengthways and cut one half into thin slices.
Pop the onions, beetroot, potatoes and lemon slices onto a baking tray. Scatter over any remaining rosemary. Drizzle with 1 tbsp olive oil. Season. Toss to coat. Roast for 30 mins.
After the veg has cooked for 30 mins, remove from the oven. Toss the veg then place the pork fillet on top. Add the garlic clove, unpeeled, to the tray. Pop back in the oven and cook for 30-40 mins.
Check the pork is cooked through by piercing it with a sharp knife. The juices should run clear. Squeeze the garlic from its skin and mash. Fold through the ruby veg.
Juice the remaining lemon half. Pour into a mug and mix with 1 tbsp olive oil. Season. Toss through the spinach leaves.
Slice the pork fillet and serve with the ruby veg and lemony spinach.