Preheat the oven to 200°C / Fan 190°C / Gas 6.
Pour 1 tbsp of oil into a frying pan and bring to a medium heat. Pick the sage leaves and fry them in the oil for a minute, until crispy. Remove the leaves from the oil and allow to drain on some kitchen paper.
Peel the sweet potato and cut into small chunks. Peel the red onion and cut into small wedges.
Tip the sweet potato and onion into the pan with the oil used to cook the sage. Scatter with the chilli flakes (go easy with this spice as it is HOT) and season well. Toss to coat everything. Tip into an oven dish and roast in the oven for 30 mins.
Heat a griddle or heavy frying pan. Fry the whole red pepper over a high temperature for 4-5 mins on each side, till starting to blacken and blister. Halve and scoop out the seeds, then slice the flesh into long strips.
Finely slice or grate the garlic and add to a bowl. Zest the lemon and add that along with the juice. Combine into a dressing with 1 tsp of olive oil and a sprinkle of seasoning.
Dress the spinach leaves and scatter onto the bottom of each plate. Add the sweet potato, onion and pepper on top. Finish with the crispy sage leaves and an extra drizzle of dressing.