Melt butter in a pan and gently fry onion and garlic over low heat until softened but not brown. Add thyme leaves. Pour over stock and bring to bubbling. Add grated Cheddar and Maille Honey Dijon mustard and stir to melt cheese. Add bread crumbs and simmer for a few minutes to amalgamate and thicken. Season with freshly ground white pepper.
Heat oil in a pan and fry potato slices crisp. Reserve on kitchen paper. Fry remaining onion rings until caramelised and crisp. Reserve with potato slices.
Ladle soup into bowls or soup cups and decorate with potato slices with onion rings on top and extra thyme leaves scattered over.