For dough, combine yeast with 230ml tepid water in a bowl and stand until frothy (5-10 minutes). Transfer to an electric mixer fitted with a dough hook and add flour, olive oil and ½ tsp salt, and knead until smooth but still a little sticky (10-15 minutes). Cover and set aside until doubled in size (1 hour).
Meanwhile, combine onion and olive oil in a large saucepan over medium heat. Season to taste, then cover and cook, stirring occasionally, until caramelised (1 hour).
Preheat oven to 230C. Roll out dough on a lightly floured surface, dusting occasionally to prevent sticking, to a 35cm x 25cm rectangle. Place on a baking tray lined with baking paper. Spread onion mixture over, then scatter with anchovies and olives. Bake until golden and puffed (13-16 minutes). Serve warm.