Cauliflower soup with mustard and Gruyère croûtons

olive oil, plus extra to serve

gm butter, coarsely chopped

onion, finely chopped

garlic cloves, thinly sliced

large Desiree potato, finely chopped

cauliflower, trimmed into small florets

chicken stock

gm grated Gruyère

milk

gm butter, melted

olive oil

Dijon mustard

grated Gruyère

gm day-old sourdough, coarsely torn