Source: FOSS Cooking (community recipes)
Sweat the onion in butter over medium heat until it turns translucent and loses its bite — about 5 minutes. This breaks down the sulphur compounds that would otherwise dominate the finished dish. Set aside.
Slice the potatoes to 3 mm thickness; thinner than 1 cm matters here because you're building a casserole-cooking where the potato must soften through to the centre while the surface crisps. A mandoline is the fastest tool. Cut the ham into 1 cm strips — wide enough that they don't dissolve into the cream, narrow enough to distribute seasoning evenly through the layers. Pat the potatoes dry with a cloth; excess moisture will steam rather than soften.
Layer the potatoes and ham in a 2-litre baking dish, starting with a third of the potatoes, then ham, then onion, repeating until you finish with potato. Pour the cream evenly over the stack — it should reach about three-quarters up the ingredients. Season with salt, black pepper, and a quarter-teaspoon of cayenne; the heat won't be aggressive, but it will sharpen the cheddar's richness. Strip the thyme leaves from their stems and scatter them through the layers. Crown with the grated cheddar, distributed evenly to catch the heat uniformly.
Bake at 200°C uncovered for 50 minutes. At this point, the surface should be golden and the potatoes knife-tender at the centre — test by piercing to the bottom of the dish. The edges will bubble slightly. Cover loosely with foil and continue for another 20 minutes to prevent the cheese from burning while the innermost potatoes finish cooking. The cream will have reduced by roughly half and thickened through potato-cooking|starch release and the residual heat of the baking dish.
Rest for 5 minutes before serving. This allows the casserole to set enough that portions hold their shape. The ham's salt and smoke will have infused the cream; you're not serving separate components but a unified gratin.
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