In a large mixing bowl, stir the flour and grated sweet potatoes to coat the sweet potato pieces.
Add the olive oil, thyme, salt, garlic, yeast and lukewarm water.
Stir with a wooden spoon until it becomes too difficult (the mixture will be rough and shaggy), then turn the dough out onto a lightly floured work surface and knead until it becomes smooth and elastic, about 5 minutes.
Return the dough to the bowl (no need to wash), cover the bowl with plastic wrap and let the dough rise on the counter for an hour, until doubled in size.
After the first hour, punch the dough down to deflate it, cover the bowl and let rise for 45 minutes, again until doubled in size.
Turn the dough out onto an 11 × 17-inch (28 × 43 cm) parchment-lined baking tray and spread the dough to fill it.
Cover the dough with a tea towel and let rise for 30 minutes.
Preheat the oven to 400°F (200°C).
Uncover the dough and use your fingertips to dimple the focaccia.
Bake for 40 to 45 minutes, until the focaccia is a deep golden brown.
Immediately from the oven, brush the hot bread with olive oil and sprinkle with a little chopped thyme and sea salt.
Cool the focaccia in the tray on a rack for at least 30 minutes.
