step 1
Heat oven to 200C/180C fan/gas 6. Pour the oil into a baking dish, about 20 x 30cm, and put in the oven for 10 mins to heat up.
step 2
While the oil heats, put the flour in a large bowl and make a well in the centre. Beat together the milk and eggs, and pour into the well gradually, whisking all the time, until a batter is formed. Season and stir in the sage.
step 3
Remove the dish from the oven, carefully pour in the batter and top with the thick slices of mushroom. Return to the oven for 15-20 mins until the batter is golden and puffy, and the mushrooms are cooked.
step 4
While the dish is in the oven, mix the crème fraîche, garlic and mustard together in a small pan. Heat gently over a very low heat so it doesn’t split, season and serve on the side.