Combine the flour and salt in a large bowl and make a well in the centre. Add the yeast and oil to 140ml warm water and stir to combine. Pour the liquid into the well and stir until the mixture starts to come together into a dough. Remove from the bowl and knead for 2-3 minutes, until the dough is smooth and pliable.
Grease a bowl with oil and place the dough in the bowl. Cover with clingfilm and leave in a warm place for 45 minutes until doubled in size.
Meanwhile, heat the oil in a saucepan over a low heat. Add the onion and garlic and sweat gently for 6-8 minutes. Add the tomatoes and a few sprigs of thyme and stir well. Partially cover and leave to simmer for 15-20 minutes, until reduced and thick. Remove the thyme sprigs and season with black pepper. Purée with a stick blender or in a mini processor, then leave to cool slightly.
Preheat the oven to 220°C/gas mark 7. When the dough has doubled in size, tip on to a floured surface and knock out the air. Divide in half and shape into two balls. Roll the balls into two large 12-inch rounds, then place each on a separate baking tray. Using your hands, shape into heart shapes.
Working from the top, cut each pepper into four flat slices, discarding the core in the middle. Using a heart cutter, cut out as many heart shapes as you can. Using a really sharp knife, cut the hearts horizontally to make them really thin. Alternatively, you can use pre-roasted peppers from a jar and cut into shape.
Spread the bases with the tomato sauce, then top with mozzarella cheese and scatter over the pepper hearts. Bake in the oven for 12-15 minutes.