Heat your oven to 200°C/Fan 180°C/Gas 6. Put a large roasting tin in the oven to heat up.
Scrub your potatoes and cut them into 1cm-thick slices. Cut the mushrooms into slices that have a similar width. Pick the rosemary leaves off their stalks. Toss the potatoes, mushrooms and rosemary into the roasting tin with 1 tbsp olive oil and a good pinch of salt and pepper. Roast for 20 mins.
While the veg roast, peel and thinly slice the garlic. Grate the zest off the lemon.
When the veg have roasted for 20 mins, add the garlic and lemon zest with 1 tsp fennel and a pinch of dried chilli flakes (these are spicy, so use as much or as little as you prefer). Drizzle over 1 tbsp freshly squeezed lemon juice. Slide back into the oven for a further 5 mins, or till the potatoes are golden and crisp.
While the veg finish roasting, whisk the balsamic vinegar with 1 tbsp olive oil and 1 tbsp freshly squeezed lemon juice. Season with a pinch of salt and pepper.
Fold the rocket through the roast veg. Drizzle the dressing over the top and serve warm.