Roast the potatoes: Preheat your oven to 200°C (180°C fan). Prick the potatoes all over with a fork. Rub them with the olive oil and a generous pinch of salt. Place them directly on the oven rack and bake for 50-60 minutes, until the skins are crisp and a knife slides through the centre with no resistance. The smell of baking potato skins will fill your kitchen with a promise of good things to come.

Sweat the vegetables: While the potatoes roast, melt the butter in a large stockpot over a medium-low heat. Add the chopped onion and celery. Cook for 8-10 minutes, stirring occasionally, until the onion is translucent and smells sweet, not burnt. The celery should have softened, offering a gentle, earthy aroma. Add the garlic and cook for another minute until fragrant.

Build the roux: Sprinkle the flour over the softened vegetables. Stir constantly for 2 minutes. You'll see the mixture turn into a pale, bubbling paste. This step is key — it cooks out the raw flour taste and ensures a silky, rather than gritty, soup. The sound of it sizzling gently is a good cue.

Whisk in the liquids: Pour in the stock a little at a time, whisking vigorously after each addition to prevent lumps. The mixture will thicken quickly at first. Once all the stock is incorporated, switch to a wooden spoon and add the milk. Bring the soup to a gentle simmer — you'll see lazy bubbles breaking the surface — and cook for 5 minutes. It should look pale and slightly thickened.

Prepare the potatoes: Once the potatoes are cool enough to handle, slice them in half lengthways. Scoop the fluffy flesh into a bowl. Chop the skins into small, 1cm pieces. The skins will add wonderful texture and a concentrated baked-potato flavour.

Combine and simmer: Add the potato flesh and chopped skins to the pot. Stir everything together. Bring the soup back to a gentle simmer and cook for 10 minutes, stirring occasionally. The soup will thicken further and the potato pieces will begin to break down, creating a wonderfully textured, creamy base. The colour will deepen to a warm, buttery yellow.

Finish with cream and cheese: Remove the pot from the heat. Stir in the double cream, grated cheddar, and sour cream. Keep stirring until the cheese has completely melted and the soup is a smooth, cohesive whole. Taste and adjust the seasoning with salt and pepper. It should be rich, savoury, and deeply comforting.

Serve immediately: Ladle the soup into warm bowls. Top with a generous swirl of extra sour cream, a shower of snipped chives, and a final grating of cheddar. The contrast between the hot, creamy soup and the cool toppings is a joy.