Heat your oven to 200°C/Fan 180°C/Gas 6. Pull the leaves off the cauliflower and break it into smallish florets. Chop the stalks into chunks. Pop in a bowl, drizzle with olive oil and season with salt and pepper.
Spread the cauliflower out on a baking tray or roasting tin. Roast for 35 mins till the cauliflower is tender and a little golden brown.
While the cauliflower cooks, peel and finely slice the onion. Finely grate or pare the zest from the lemon. Juice it. Pick the leaves off half the sage sprigs. Finely slice them. Rinse the capers.
Fill a kettle and boil it. Warm a dry frying pan over a medium heat. Add the breadcrumbs to the pan. Toast the breadcrumbs for 2 mins till they’re golden. Keep stirring them while they cook. Tip into a bowl.
Warm a drizzle of oil in the frying pan. Add the onion. Season with salt and pepper. Keep the heat lowish and gently fry the onion for 10-15 mins till they are golden and caramelised.
Fill a pan with hot water from the kettle. Add a little pinch of salt. Add the penne. Simmer for 8 mins till the pasta is cooked but not too soft. Drain the pasta.
Add the breadcrumbs, penne, cauliflower, lemon zest and juice, capers and most of the sage to the pan. Give it a stir and warm for 2 mins. Taste and adjust the seasoning. Serve topped with the remaining sliced sage.