Winter root mash with buttery crumbs

Name: Winter root mash with buttery crumbs
Prep time: 1 h
Cook time: 40 min
Total time: 1 h 40 min
Cuisine: British
Ingredients: 650g parsnips , cut into even chunks; 650g swede , cut into same size chunks as the parsnips; 142ml tub soured cream; 1 rounded tbsp hot horseradish (English Provender is good); 2 tbsp fresh thyme leaves; butter , for greasing; 50g butter; 1 small onion , finely chopped; 50g fresh white breadcrumbs (from about 4 slices); a small handful thyme leaves, plus extra for scattering; 25g parmesan , coarsely grated