For the dough:
1) Whisk together the warm water, yeast, sugar, and melted butter in a small bowl; let stand until yeast is dissolved, about 10 minutes.
2) Whisk 155g flour and salt together in a medium bowl. Add yeast mixture to flour mixture and stir with a wooden spoon until mixture clumps together. Mix until incorporated. Add more flour, a little at a time, if dough is too wet.
3) Turn dough out onto a lightly floured work surface and knead until smooth and elastic, about 3 minutes. Divide dough into 12 equal pieces; roll each into a ball. Place on a flour-dusted baking tray; cover with cling film. Let rise in a warm place until doubled in size, about 45 to 60 minutes. (Dough can be made up to this point 1 day ahead. Keep covered and refrigerate.)
For caramelised onions:
1) Heat oil in a large nonstick heavy saucepan over medium-high heat. Add onion slices to the saucepan and season with 1/2 tsp salt. Cook onions without turning until they are softened and beginning to brown, about 5 to 7 minutes. Add sherry vinegar and honey and stir well to incorporate; reduce heat to medium and continue to cook until onions are tender and brown, occasionally stirring gently, about 10 minutes. Season, to taste, with salt.
2)Remove saucepan from heat and cool. (Onions can be prepared up to 1 day ahead. Cover and refrigerate.)
For the assembly:
1) Preheat oven to 230 degrees C/Gas 8.
2) Sprinkle 2 baking trays lightly with cornmeal. Roll out each dough ball to 7.5 to 10-cm rounds. Transfer 6 rounds to each prepared baking tray. Pierce dough all over with fork. Brush generously with extra-virgin olive oil. Arrange onions evenly atop each round, leaving .5-cm plain border. Top with thyme leaves. Bake flatbreads until crusts are puffed and golden, about 12 to 15 minutes. Sprinkle crumbled goats cheese on top of each mini focaccia. Transfer to serving platter and serve immediately.