Heat the butter and olive oil in a large frying pan over medium heat. Add the onion and cook for 4 to 5 minutes until it turns translucent and smells sweet, not sharp.

Add the garlic and cook for 30 seconds until fragrant. You’ll know it’s ready when the aroma hits you immediately and the garlic just begins to turn golden at the edges.

Tip in all the mushrooms. Cook for 6 to 8 minutes, stirring occasionally, until they release their liquid and shrink down. They should look glossy and deeply browned, not watery.

Sprinkle the flour over the mushrooms and stir constantly for 1 minute. The flour will coat everything and the mixture will look slightly dry and pasty — that’s exactly right.

Pour in the vegetable stock, soy sauce, Dijon mustard, smoked paprika, and dried thyme. Stir well, scraping up any browned bits from the bottom of the pan. The sauce should smell smoky and rich.

Transfer everything to the slow cooker. Cover and cook on low for 4 hours or on high for 2 hours. The sauce will thicken and darken slightly, and the mushrooms will be meltingly tender.

About 10 minutes before serving, stir in the sour cream. The sauce will turn a pale, creamy tan colour. Taste and adjust the salt and pepper — it usually needs a good pinch of both.

Sprinkle over the fresh parsley just before serving. It adds a bright green pop against the creamy sauce and a fresh, grassy note.