Put the butter in a bowl, add the garlic and mix well. Spoon the butter out onto a sheet of cling film and roll up to make a sausage-shaped log. Chill for 10 mins.
Heat oven to 200C/180C fan/gas 6. Slice the baguette into about 12 slices but not all the way through, leaving the base intact to hold it together.
Remove the cling film from the butter and thinly slice on a chopping board. Press each butter slice between the slices of bread.
Wrap the baguette in foil, place on a tray and bake for 5–6 mins, then peel back the foil. Cook for a further 4–5 mins to crisp up.