Heat the olive oil and butter in a large, wide sauté pan over medium-low heat.
Add the leeks with a good pinch of salt and cook gently for 15 - 20 minutes, stirring often, until very soft and glossy. They will be starting to color slightly.
Set aside in a bowl and add a knob of butter to the pan if it is dry. Turn the heat up to medium-high.
Add the sausage meat and cook for 5 minutes, pressing it into the pan in larger pieces and letting it sit so it can develop some color. Once the underside is golden, turn the pieces over and cook until browned.
Lower heat slightly, then add the garlic, chilli flakes, and thyme/rosemary. Cook for 30 seconds until fragrant.
Add the orzo to the pan and stir to coat, cooking for 1-2 minutes.
Add the chicken stock and bring to a very gentle simmer, adjust heat as needed. It should just be at a very delicate bubble.
Cook for about 10 minutes, stirring often so the orzo doesn't stick, until it is just tender and the sauce has thickened.
Add the cream, leeks, lemon juice, and pepper and mix well. Cook gently for a further 2–3 minutes until glossy. The orzo should look slightly loose.
Sprinkle the grated parmesan on top and mix through to melt. Taste for seasoning and serve immediately.