Heat your oven to 180°C/Fan 160°C/Gas 4. Peel and thinly slice the onion. Lay on a baking tray. Halve the pepper, scoop out the seeds and thickly slice. Arrange on the tray and drizzle over ½ tbsp olive oil. Season. Drop on the unpeeled garlic clove. Roast for 20 mins.
Snap off the woody ends from the asparagus. When the pepper and onion have cooked for 15 mins, remove the garlic from the tray. Place the asparagus on the tray. Toss with the rest of the veg. Slide back into the oven for 10 mins till charred.
Bring a large pan of salted water to a rolling boil. Drop in the spaghetti and cook for 8 mins.
While the spaghetti is cooking, finely chop the wild garlic leaves. Place in a bowl. Zest in the lemon. Squeeze in the juice from half the lemon. Finely grate half the Cheddar and stir that in.
Using the back of the knife, squeeze the roasted garlic flesh from its papery skin. Add to the bowl. Drizzle in 1 tbsp olive oil. Mix well to make a chunky pesto sauce. Taste and season.
Drain, reserving a cupful of water. Tip back into the pan and toss though the wild garlic pesto. Add a few tbsp of the cooking water if you want to loosen the sauce.
Divide the spaghetti between 2 plates. Top with the roast tomatoes, onion and asparagus. Scatter over the parsley leaves and serve with wedges of lemon and the remaining Cheddar for grating.