heat to low and cook slowly for 20 to 25 minutes, stirring regularly, until the onions are very soft and lightly golden. Do not let them brown.
Stir through the chopped anchovies and season with a little pepper. Remove from the heat.
Preheat the oven to 180°C (350°F).
Slice a third off the top of each baguette lengthways so the base is slightly thicker than the top. Press your fingers into the cut surface to semi-hollow out the bread slightly. This helps the onion mixture sit in the baguette rather than sliding off.
Divide the onion mixture evenly between the two baguettes and spread in a generous layer. Lay 3 whole anchovy fillets across the top of each baguette in a criss-cross pattern, or cut thicker fillets in half lengthways and use 6 halves per baguette. Dot the olives along the top.
Bake for 20 to 25 minutes until golden and crisp.
Serve hot with a crispy green salad, or allow to cool and serve at room temperature.