Cook parsnips in a pan of boiling salted water for 2 mins, then tip in the celeriac and boil together for 8 mins. Drain well, then mash with butter and cream. Season, then add a grating of nutmeg, stir in the sage and spoon into a buttered ovenproof dish. Make up to this stage 1 day ahead or freeze for one month. Defrost in fridge overnight.
Heat oven to 190C/fan 170C/gas 5. Mix breadcrumbs, Parmesan, almonds and oil together. Season, scatter over the mash, then bake for 35-40 mins from cold, 25-30 mins if not, until topping is crisp and golden.