Peel the red onion and slice thinly. Peel the garlic cloves and crush or finely chop. Thinly slice the chilli on an angle (use half for a little less heat). Slice the cherry tomatoes in half. Pick the rosemary leaves and finely chop them. Roughly shred the sprouts and olives.
Fill a large pan with boiling water and set on a high heat. Sprinkle in a generous pinch of salt. When the water has returned to the boil, add the penne and cook for 8 mins, till cooked but retaining a slight bite. Drain through a sieve or colander, retaining a mug of the cooking water.
Meanwhile, pour 1 tbsp oil into a large frying pan and bring to a medium heat. Slide in the sliced onion, garlic, rosemary and chilli. Sprinkle in a little salt and pepper and fry for 2 mins, stirring occasionally.
Stir the sprouts and olives into the onions and continue to cook for a further 2 mins. Add the cherry tomatoes and cook for 3-4 mins, stirring now and then, till slightly softened.
The pasta should be cooked by now. Toss it into the vegetables in the frying pan along with the reserved cooking water. Stir well with a spoon, till the liquid thickens and coats the pasta. Squeeze in half of the lemon juice and drizzle in 1 tbsp oil. Have a taste, and add more salt and pepper if it needs it.
Scoop the pasta onto plates and serve with wedges of the reserved lemon and a good crack of pepper.