Heat your oven to 180°C/Fan 160°C/Gas 4. Peel the sweet potatoes and chop into 2cm chunks. Tumble into a baking dish. Season and drizzle with ½ tbsp oil.
Wrap the garlic cloves in a square of foil and nestle in with the sweet potato. Scatter over the rosemary leaves. Roast for 30 mins till the potatoes are soft and start to caramelise at the edges.
While the sweet potatoes cook, bring a large pan of water to the boil. Add a pinch of salt then tip in
the pasta. Simmer for 8 mins till the pasta is al dente (soft but with a slight bite). Drain, tip back into the pan and stir through half the watercress to slightly wilt it.
When the sweet potatoes have cooked, take out the garlic. Squeeze the garlic from their skins (use
a fork to press down on them as they’ll be hot). Add them back to the dish. Gently mash, leaving the sweet potatoes a little chunky.
Pour in the crème fraîche. Crumble in a quarter of the Perl Las blue cheese. Mix together. Fold in the pasta and half of the cherry tomatoes.
Top the bake with the remaining cherry tomatoes. Crumble over another quarter of the Perl Las.
Bake for 20 mins till golden and bubbling. Serve with the remaining watercress on the side.