Place the anchovies in a small dish, add the milk and leave to soak for 10 minutes to remove excess saltiness and draw out some of the oil.
Drain, rinse and pat the anchovies dry with kitchen paper. Place them in a food processor with the garlic and lemon juice and pulse to make a paste. Alternatively, mash them with a fork, or with a pestle and mortar.
Heat the toaster to high. Toast the bread on both sides, then spread a quarter of the paste over one side of each slice: it is very strong, so you only need a little.
Slice the tomatoes, discard the ends and arrange the slices on top of the anchovy paste. Drizzle with oil, sprinkle with black pepper and return to the toaster or grill for 1-2 minutes until the tomatoes are softened. Serve scattered with basil leaves.