Tear the bread into pieces and toss in a bowl with the milk and 1 tbsp of water. Leave to soak for 20 mins. Gently squeeze the bread to remove some of the excess liquid (though not all). Heat 2 tbsp of the oil in a large frying pan. Cook the bread over a medium heat, turning every so often, until dark golden brown and crisp, then turn the heat down and cook until the pieces are warm through to the middle. Season and cook a little more, turning the pieces over. It will take about 15 mins to make the migas. Heat oven to 200C/180C fan/gas 6.
Meanwhile, hold a corn cob standing up in a shallow roasting tin. Using a sharp knife, cut down the sides, removing the kernels as you work round. Toss the kernels with ½ tbsp of the oil and a little seasoning. Roast in the oven for 20 mins, tossing after 15 mins. It should be scorched a little at the edges.
To make the dressing, put the mayonnaise, mustard, garlic and chives in a bowl, then gradually whisk in the buttermilk. There shouldn’t be any lumps of mayo left.
Halve and stone the avocadoes and cut each half into slices. Peel the skin from each slice. Put the avocado in a broad shallow serving bowl and squeeze the lime juice all over them. Turn over in the juice and season. Add all the other salad ingredients and toss together. Spoon some of the dressing over and serve the rest on the side.