Place yeast, sugar and 1 cup warm water in a jug. Whisk with a fork until yeast has dissolved. Cover. Stand in a warm place for 10 minutes or until frothy.
Sift flour into a large bowl. Make a well in the centre. Add yeast mixture, 2 teaspoons salt and 1 tablespoon oil. Mix to form a soft dough. Turn out onto a lightly floured surface. Knead dough for 10 to 15 minutes or until smooth and elastic. Place in a large, lightly greased bowl. Cover with lightly greased plastic wrap. Set aside in a warm place to prove for 1 hour or until doubled in size.
Meanwhile, heat 1 tablespoon oil in a large frying pan over medium-low heat. Cook onion, stirring occasionally, for 15 minutes or until softened and beginning to caramelise. Set aside.
Brush a 20cm x 30cm Swiss roll pan with 2 teaspoons oil. Using your fist, punch down dough. Turn out onto a lightly floured surface. Knead until smooth. Press into prepared pan. Cover with a damp tea towel. Set aside in a warm place for 30 minutes or until doubled in height. Preheat oven to 200C/180C fan-forced.
Using fingertips, press holes into the dough. Brush with remaining oil. Sprinkle with onion, walnuts, rosemary and remaining salt. Bake for 25 to 30 minutes or until golden. Stand in pan for 5 minutes. Transfer to a baking paper-lined wire rack. Cool. Serve focaccia warm or cold.
Recipe courtesy of taste.com.au
Serving size 4