Note: this recipe can be made with more or less flour/potato, as long as you use equal amounts: just adjust amount of sourdough/salt accordingly. I tend to weigh my potatoes pre and post mashing, and then add equal amount of flour. 500g as below will make 1 large and 1 smaller focaccia.
If you do not have starter, just use a sachet of dried yeast disolved in warm water (circa 250ml) with a tsp of sugar.
500g plain or all-purpose/00 flour
500g mashed potatoes
Sourdough discard (or starter) - 1 cup
1.5 tsp salt
Optional 1/2 tsp dried yeast for a quicker rise
Water, if needed
Small tomatoes, halved
Weigh potatoes. If large, boil in skin(s). Once cooled, peel and then mash. I usually reserve a bit of water for the dough mix. Once cooled, weigh again then add equal amount of flour. Mix well together. Add salt.
Add enough starter/discard to make a dough. A cup is usually enough. If needed, add yeast. If water needed, add a drop.
Knead to a smooth dough - usually 10 minutes. Depending on size of baking trays, divide and plop dough into an well oiled dish. Gently squash down/shape according to size of tray.
Cover with a damp cloth and leave somewhere warm to rise. If using dried yeast, 1.5-2hrs should be sufficient. If using starter only, will need a longer rise or prepare/leave overnight to rise.
When risen, lightly squash down with fingers, if required, to flatten out again.
Switch oven on to 200c
Prepare topping. Heat 2tbsp olive oil in a small saucepan. Add halved tomatoes, capers, seasoning and oregano. Gently heat up until tomatoes are softened.
Pour/spread topping over dough, spreading out as necessary. Ensure all surfaces of dough are covered in the tomato/oil mix. If not, drizzle more oil over. Sprinkle more oregano over.
Bake 30 mins circa, depending on size of trays. If using glass, the focaccia is cooked when underneath is golden in middle.
Note: the above is for the classic focaccia Barese, originating from Bari in Puglia. I also like to add sliced red onion and pepper slices to mine, which I cook in the saucepan with the tomatoes.