Preheat your oven to 350f/180c.

Put the butternut and onion pieces on a baking sheet lined with parchment paper and drizzle over the olive oil. Sprinkle over a good pinch of salt and pepper.

Roast in the oven for 40 - 50 mins until lightly charred (see the pictures above).

You might need to take the onion pieces out earlier so watch them closely.

Pureé with a stick blender or in a food processor with 1 cup water. 

Cook your pasta in a pot of well salted boiling water. Reserve 1 cup of pasta water before draining.

Melt the butter over medium heat and when just sizzling add the sage leaves.

Cook, swirling the pan so the butter browns evenly. Remove the sage leaves when crisp and the butter is lightly browned. 

Set the leaves aside on a paper hand towel and leave butter in pan.

Add the garlic to the butter and sizzle for 30 seconds, moving it around the pan so it doesn't burn. Turn the heat down if you need.

Add the tomato paste and let it gently sizzle for 2 minutes, whisking it into the garlic and butter.

Lower heat and add the cream and ½ cup pasta water and mix well. I like to use a flat whisk. 

Bring it to a really gently simmer here, it shouldn't be bubbling aggressively.

Add the butternut puree and mix in well.

Add the parmesan in two separate lots, whisking and letting it melt between each addition.

Add the lemon juice and black pepper and mix in well. Season to taste with salt.

Add your pasta straight into the sauce and let it gently bubble away for a minute or so.

Add more pasta water to loosen the sauce if needed. This is quite a thick sauce so loosen as much as you need.

Serve with extra parmesan, a good crack of black pepper and the crisp sage on top.