Brush the fleshy side of the spaghetti squash with olive oil and season well. Turn it flesh side down onto a baking tray and bake at 375 for 45 minutes to an hour, depending on the size of your squash.
Remove and gently fork the squash so that it forms noodle-like strips. Transfer these to a bowl and place back in the oven to keep warm.
Melt the butter in a saucepan, add the garlic and leek and saute until the leek is soft.
Take the pan off the heat and flake in the mackerel.
Remove the squash from the oven and place in the saucepan together with the leeks and mackerel. Add the ricotta and gently stir everything together until well combined. Season with black pepper and serve immediately.