Preheat oven to 180C. Place the ingredients for the crumble into a small food processor and pulse to form a rough crumb.
Place onto a baking paper lined baking tray and bake until golden, about 15 minutes. Set aside.
Thinly slice the potatoes and sweet potato (easiest on a mandolin) and place into separate medium bowls.
Add 2 tbsp olive oil, 2 tbsp cream and 1 tbsp thyme to each bowl, season with salt and pepper and mix well.
Rub 1 tbsp oil onto the surface of a 20cm round baking dish. Place the potato and sweet potato in alternating rows, stacked on their sides.
Pour over the stock, cover with aluminium foil and bake for 45 minutes.
Remove foil, drizzle over 1 tbsp olive oil & parmesan and return to the oven for 10 minutes to brown.
Serve gratin topped with the crumble mix.
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