Melt butter in a casserole over medium heat, add shallot and garlic and stir occasionally until very tender (4-5 minutes). Add bacon bones, celeriac, potato, stock, milk and herbs, bring to the boil, reduce heat to low, partially cover with a lid and simmer until vegetables are very tender (45-50 minutes). Remove bacon bones and herbs, season to taste, then process with a hand-held blender until very smooth. Stir through Raclette until melted and combined, keep warm.
For Raclette croûtons, preheat a grill to high. Place baguette slices on an oven tray and lightly toast one side. Rub toasted sides with garlic, turn and scatter over Raclette. Toast until bubbling and golden (2-3 minutes) and serve immediately with celeriac soup topped with chopped herbs, if using.