Peel and roughly chop the Floury Potatoes into even pieces and place them in a large pan of cold water; bring to a boil and simmer until very tender, about 15–20 minutes.
Drain the potatoes well, return to the pan off the heat and mash with a generous knob of Unsalted Butter and a splash of Whole Milk until smooth and creamy; season to taste with Sea Salt and freshly ground Black Pepper. Keep warm.
While the potatoes are cooking, heat a tablespoon of Sunflower Oil in a large frying pan over medium-high heat and add the Cumberland Sausage. Cook, turning occasionally, until deeply browned and cooked through (about 12–15 minutes). Remove the sausages to a plate and keep warm.
Reduce the heat to medium, add a little more Unsalted Butter to the pan if needed and add the sliced Onion. Cook gently until soft and golden, about 8–10 minutes.
Sprinkle in the Plain Flour and stir for 1–2 minutes to cook the raw flour taste, then gradually pour in the Beef Stock, whisking to remove lumps until you have a glossy gravy.
Stir in a splash of Worcestershire Sauce and season the gravy with a pinch of Salt and more Black Pepper to taste; simmer for a few minutes to thicken and deepen the flavor.
Return the browned Cumberland Sausage to the pan with the gravy for a couple of minutes to reheat and coat in sauce.
Meanwhile, cook the Frozen Garden Peas in boiling water for 2–3 minutes until just tender, then drain and toss with a small knob of Unsalted Butter and a light pinch of Salt.
Serve the sausages and onion gravy over a mound of creamy mash, with the buttered peas at the side. Enjoy immediately while hot.
