Prepare the beef: Pat the beef chunks dry with kitchen paper — this helps the flour stick. In a large bowl, toss the beef with the flour, salt, and pepper until each piece is lightly coated. You'll see a dusty, pale coating on all sides.

Brown the beef: Heat the olive oil in a large frying pan over medium-high heat until it shimmers and a piece of beef sizzles instantly when added. Working in two batches (to avoid overcrowding), brown the beef for 2-3 minutes per side until each chunk is deeply golden-brown and smells rich and savoury. Transfer the browned beef to the slow cooker insert.

Sauté the aromatics: In the same pan (no need to wash it), add the diced onion and cook over medium heat for 4-5 minutes, stirring occasionally, until the onion turns translucent and softens. Add the minced garlic and sliced mushrooms, and cook for another 3-4 minutes — the mushrooms will release their liquid, which should sizzle and then reduce, leaving them a glossy, dark tan colour. Scrape everything into the slow cooker.

Deglaze the pan: Pour a splash of the beef stock into the hot pan and stir vigorously with a wooden spoon, scraping up all the browned bits stuck to the bottom — these are packed with flavour. Pour this liquid into the slow cooker.

Add liquids and seasonings: Pour the remaining beef stock into the slow cooker, then stir in the tomato purée, Worcestershire sauce, and smoked paprika. Give everything a good stir — the liquid should look a deep, rusty red and smell earthy and savoury.

Slow cook: Cover the slow cooker with the lid and cook on low for 6-8 hours, or on high for 3-4 hours. After 6 hours on low, the beef should be fork-tender — when you press a piece with a fork, it should break apart easily without resistance. The sauce will have darkened to a rich mahogany colour.

Finish the sauce: In the last 15 minutes of cooking, stir the cornflour slurry into the slow cooker. Let it cook, uncovered, for 10 minutes — you'll see the sauce thicken from a thin broth to a glossy, velvety consistency that coats the back of a spoon.

Add the soured cream: Turn off the slow cooker (or switch to 'keep warm'). Stir in the soured cream until fully incorporated — the sauce will turn a pale, creamy beige and smell wonderfully tangy and rich. Taste and adjust salt and pepper if needed.

Rest and serve: Let the stroganoff sit for 5 minutes before serving. This short rest allows the flavours to meld together. Garnish with fresh parsley and serve over egg noodles, rice, or mashed potatoes.