Step by step
Preheat oven to 180°C. Grease a 30cm diameter (3.25-litre capacity) round ovenproof dish with butter.
Heat 1 tbsp olive oil in a frying pan over medium-high heat.
Add 3 chopped rashers rindless smoked bacon and cook, stirring, until crisp
(4-5 minutes). Add 1 chopped garlic clove and 2 tbsp finely chopped thyme; stir until fragrant. Set aside.
Place 1.25 litres pouring cream in a large bowl; season generously with salt and freshly cracked black pepper. Peel 2kg medium Dutch cream potatoes then thinly slice on a mandoline straight into the cream.
Layer potatoes in prepared dish, scattering bacon mixture, 1 thinly sliced onion and 80gm finely grated parmesan between layers and seasoning well as you go. Pour cream over the top to cover (you may not need it all) and dot with 40gm diced cubes of butter.
Place dish on an oven tray lined with baking paper. Bake until golden brown and bubbling (1 hour).
Trim crusts from a 350gm piece of stale sourdough, then tear in coarse breadcrumbs over a bowl. Add 40gm melted butter and ½ tsp smoked paprika; season then scatter over gratin. Top with a little extra grated parmesan and thyme leaves. Continue to bake until crumbs are golden (6-8 minutes), then stand for 10 minutes before serving.